"It just felt incredible," Reuben said recently in a phone interview from Utah, where she was attending a fundraiser for the Waterkeeper Alliance, an environmental advocacy group for which she serves on the board of trustees.

The actress, who joined "ER" in its second season, added: "Of course, there was a lot of apprehension and anxiety, like returning home after not having been there for so many years. I didn't know what to expect. But when I got there, it was like no time had gone by at all." But much has transpired since she last walked the halls of the fictional Chicago-based County General Hospital. "ER," which just celebrated its 300th episode, is struggling in its 14th season, and some industry insiders predict that the series could reach the finale this year despite a recent infusion of new cast members that includes John Stamos.

And there were few familiar faces to greet her. None of the key cast members who costarred with her were around. In addition to Reuben, the show helped launch the careers of Julianna Margulies, Anthony Edwards and Noah Wyle, and made George Clooney a star.

But she was fueled by her memories. Jeanie Boulet was a first for a prime-time series network television -- a woman stricken with HIV determined to continue with her life and career despite the stigma surrounding the virus. She contracted it from her estranged husband, who had had unprotected sex with another woman.

Easing into the advanced stages of the Boulet character was a breeze, Reuben said. "It really felt kind of great to step back into the shoes of Jeanie. There's no better thing for an actor to ask for than to have the opportunity to step back in time and yet have new dimensions to play."

There's also a personal synergy at play: Her return coincides with Reuben's continued work as an AIDS activist. In the last few years, Reuben has spoken at World AIDS Days. This year, she produced a Showtime project, "Positive Voices: Women and HIV," in which she interviewed several women who are living with or working with HIV/AIDS, and she costarred in HBO's "Life Support," which starred Queen Latifah as a woman with the virus.

When Boulet's character departed "ER," she was still adjusting to her HIV status. The new episode finds her as a divorced mother, running two AIDS clinics. When her son injures himself in a fall in gym class, Boulet winds up back in the ER, instantly clashing with Dr. Gregory Pratt (Mekhi Phifer), whom she doesn't trust. She eventually learns devastating news about her child.

Reuben's return was sparked by a recent conversation with "ER" executive producer John Wells. "We were catching up on things, and I was talking about my HIV activism in the last few years," she said. "We realized we had really done some great stuff on the show in terms of bringing this to the forefront of people's lives. In recent years, that issue seems to have slipped off the radar, so it's great to get back into it again."

Producer Christopher Chu- lack said that Reuben's comeback was "pleasantly odd. I can't think of another situation where you're around after 10 years and you get to revisit something. It was just so great to see her and see how she has matured. The character has matured like she has."

Though Reuben seems largely unchanged physically from her "ER" days, the plot surrounding Boulet is typically heavy and emotional. It's that intensity, she said, that largely prompted her departure from the drama.

Said Reuben: "In 90% of the episodes I was in, there was always intense stuff going on. That might not be the healthiest thing for an actor. When I'm in a part, I go to the places that are necessary, and because it was so impactful, it was not something I could just leave on the set. So I was burned out, emotionally exhausted, and I had to take a step away from it for a few years."

She said people were confused when they heard that her first post-"ER" gig would be as a background singer and dancer touring with Tina Turner.

"There were folks who thought I left the show to go on tour," Reuben said. "But when I left the show, I really didn't know what was going to happen next. Then I met Tina. I know when creatively I can't stand still just for security or financial reasons. I thought, 'This is crazy and fun and wicked.' Five costume changes! It was fantastic, and I would do it again."

The singing bug is still with her. Reuben has recorded a solo album, "Just for You," and co-wrote and sang the theme song for her Lifetime 2003 series, "1-800-MISSING." She has not really kept up that much with "ER" ("I catch it now and then, but it's way too intense for me," she said with a laugh) and doesn't own any of the DVD compilations ("I hardly watch any television except for the NFL and Discovery Channel"). She may run across an episode when traveling abroad: "There I'll be, speaking Italian. It's way, way too weird to see this younger version of me. . . . I just turn it off."

In addition to her activism, Reuben is juggling a diverse slate of projects. She's performing in "Conversations in Tusculum" in March at the Public Theatre in New York. She's also filmed a drama pilot produced by Steven Bochco, and she is the executive producer of a new film, "Padre Nuestro," written and directed by her boyfriend, Christopher Zalla.

She's also so pleased with her return to "ER" that she is looking forward to seeing her episode. "After all this time, there hasn't been anything close to what we did with Jeanie. I'm not one to watch my work, but I'm really looking forward to this episode."