LOS ANGELES - It was probably inevitable that Shonda Rhimes would create “Grey’s Anatomy,” ABC’s runaway hit about the romantic entanglements and complicated lives of surgeons and their overworked interns at a Seattle hospital. For two years during high school, she worked as a candy striper.

“I loved that job,” she recalled the other day. “I’m perfectly comfortable in hospitals.” She also admits to having a passion for shows about medical procedures: the pressure, the blood, the scalpels, she finds it all intriguing. “I love to watch all those surgeries on the Discovery Channel and the Learning Channel,” she said. “I’m a surgical junkie.”

Apparently Ms. Rhimes is not the only one. Her show’s mixture of medicine, drama and sex has proved such a winning formula that more than 25 million viewers tuned in last week to watch its season premiere.

Since the show’s debut in 2005 it has been one of television’s top-rated series, and for this the network rewarded Ms. Rhimes with a two-year, $10 million deal. But the biggest sign of the show’s success is ABC’s decision to move it from Sunday nights following “Desperate Housewives” to Thursday nights, where it is now going head to head with CBS’s popular crime series “CSI,” whose season premiere last week drew three million fewer viewers than “Grey’s Anatomy.”

“It’s an amazing vote of confidence, and I was thrilled that the guys at ABC think that we can anchor a night,” Ms. Rhimes said.

Her accomplishment is particularly noteworthy in a field that is still dominated by white males, said Ron Simon, a curator at the Museum of Television and Radio in New York. “She’s the only black woman show runner on a dramatic show at this point on the major networks,” he said. “It’s a tremendous achievement for a woman, African-American or otherwise.”

At her office in the residential Los Feliz neighborhood, where lawns are perfectly manicured and homes are tastefully understated, Ms. Rhimes, 36, sat in front of a computer tweaking a script for a future episode. Posters of some of the films Ms. Rhimes had written before coming to television hung on the walls: “Princess Diaries 2: Royal Engagement”; “Crossroads,” a coming-of-age movie starring Britney Spears; and the television biopic “Introducing Dorothy Dandridge,” which won Halle Berry both an Emmy and a Golden Globe.

Ms. Rhimes admitted that she was a bit surprised by the popularity of “Grey’s Anatomy,” which revolves around a surgical intern, Meredith Grey (played by Ellen Pompeo), and her on-again off-again love affair with Derek Shepherd (Patrick Dempsey), a neurosurgeon nicknamed Dr. McDreamy.

“I knew that I really liked it, my friends liked it, my family liked it, but it never occurred to me that stuff I came up with at home in my pajamas people would respond to,” she said, adding that fans often approach her in the grocery quoting lines from the show. “It’s all still surreal to me.”

She says that writing for television is more demanding than writing for the movies. “Before, I would write one or two movies a year, which means I’d work two months out of the year,” she said. “Television is 24 hours, seven days a week, and I never thought I’d like something like this, but I love it. It’s such an adrenaline rush.”

She said that learning to collaborate with a team of writers was an adjustment. On the show’s official blog, greyswriters.com, Krista Vernoff, a writer, recalled Ms. Rhimes’s first day on the set. “There was, of course, Shonda who had never been in a writer’s room before and who lurked outside the door, brooding and disturbed like maybe we were all vampires who would eat her soul if she stepped foot inside.”

Ms. Rhimes acknowledged her initial trepidation. “The concept of sitting in a room with a bunch of people and spilling out the ideas in my head seemed a little exhausting, and it seemed antithetical to the process by which I work, but now it doesn’t feel that way at all,” she said. “I became incredibly grateful for my staff of writers really quickly.”

People who work with Ms. Rhimes have noticed her changed demeanor. “She’s always been very sure of what she believes and very clear about what she wants to say, but her confidence has grown, and I’m really watching her evolve into an incredible show runner and executive producer,” said Betsy Beers, an executive producer on the show. “It’s been kind of astonishing.”

What hasn’t changed is Ms. Rhimes’s commitment to having an ethnically diverse cast, which includes a black chief of surgery, a black chief resident and Asian and Hispanic interns as well as various white characters. “We had everyone of every color read for every single part, and it was about casting the best actor in the room,” she said. “I don’t think a lot of shows do that, but it just makes sense to me.”

That diversity, she said, makes writing a bit easier for her. “If you have a show in which there’s only one character of color - which is what most shows do - then you have a weird obligation to make that person slightly saintly because they are representing all the people of color,” she explained. “But if you have all different races, people get to be good or bad, flawed, selfish, competitive.”

Growing up the youngest of six children in Chicago, Ms. Rhimes said, she preferred reading books to watching television. Her mother, a professor of education administration at DePaul University, and her father, an administrator at Ohio State University, encouraged her to read anything on the family’s bookshelf. “I read everything from Nancy Drew and ‘Secret Garden’ to ‘The French Lieutenant’s Woman’ when I was 10,” Ms. Rhimes said.

She graduated from Dartmouth with a degree in English literature and creative writing and landed a job in advertising. After a year she decided she hated it and enrolled in film school at the University of Southern California.

“I loved it immediately,” she said. “I discovered that this is what I wanted to do.” In the mid-to-late 90’s, she sold her first screenplay, “Human Seeking Same,” about a couple who fall in love through the personal ads. The movie was never made, but the mid-six-figure check she received made it possible for her to write full time.

It wasn’t until Ms. Rhimes adopted a child, now 4, that she began spending time at home and really watching television. “I realized a lot of the really good character development is happening on TV,” said Ms. Rhimes, who was huge fan of “Buffy the Vampire Slayer.” “The language was great, the world was great, and you completely invested in those characters. I’m still not over its cancellation.”

In 2002 she wrote a script for a television series that followed the lives of a group of war correspondents; it was not picked up. “They drank a lot and had a lot of sex, and the war was always sort of secondary,” she said, “so when we suddenly went to war, it seemed in poor taste to have a show about people having fun covering war.” Ms. Rhimes said she was in the process of reviving the project.

Ms. Rhimes, who once watched three films a day in the theaters, hasn’t been to the movies in three years, she said. When she does have a spare moment from “Grey’s Anatomy” (which is rare) she prefers watching television shows like “Lost,” “Weeds” and “Project Runway.”

“I’m freakishly obsessed with the American version of ‘The Office,’ ” she said.

She is fiercely protective of her show’s plot points, refusing to divulge any details about the new season. Will Meredith end up with McDreamy for good?

“I’ve always been a person who hated spoilers,” she said, shrugging. “If you already know what’s happening on a show, why are you watching it?”