Will Dr. Benton gain permanent custody of his son? Will Nurse Hathaway and Dr. Ross get back together before the birth of their twins? Do Mark and Elizabeth really have a thing for each other?

Actually, there's a more compelling question surrounding "ER" these days: How much does America care anymore?

As it begins its sixth season tonight (Channel 4, 10 p.m.), the most popular TV drama of the decade seems in need of doctoring, stat. Hard-core fans were already huffing about the malaise that set in after George Clooney (Dr. Ross) checked out last season. Now comes more flux at County General Hospital. Six new regular and recurring characters (including new doc Alan Alda, in a guest-starring role) will become part of the blood-spurting and bickering this season, and some continuing vets (Gloria Reuben's Jeanie Boulet, Julianna Margulies' Carol Hathaway) will be phased out. Clooney may reappear--or maybe he won't.

Although it seems destined to continue as the most popular series on the air, in the dog years that measure the life cycle of a TV show, "ER" is clearly approaching late middle age. The program's popularity peaked three years ago, and last season its descent began to gather speed. It averaged 25.4 million viewers a week last year, a decline of 16 percent during the season (and 33 percent during summer reruns). At its current rate of deterioration, the blockbuster show of the '90s would finish out this season with about the same total audience that a throwaway sitcom like "Grace Under Fire" commanded just five years ago.

Some of that may bespeak the changes on the show itself. Phyl Behrer, 41, who helps maintain one of dozens of Internet sites devoted to the program, complains that the writing last season was inconsistent, the character development lackluster. In part, she believes, this reflects the absence of John Wells, "ER's" original executive producer and co-creator, who has been working on other series (last season's failed "Trinity" and this season's "West Wing" and "Third Watch") but is still listed as "ER's" executive producer.

With new faces coming (Goran Visnjic as a Croatian doctor, "Homicide's" Michael Michele as a nurse), Behrer fears the show will lose even more snap. "As an avid viewer, am I excited about all this?" she asks. "Absolutely not."

But the larger story of "ER" may be about its medium. As the show slowly sinks, it's easy to see the end of an era going down with it. While some programs will match the crackling quality of "ER" in the future, it's less certain that any of them will be able to attract so large and diverse a following. With cable, with satellite TV, with VCRs and remote controls, TV audiences have splintered; the days when Americans gathered en masse to see Lucy's baby or the Fugitive's capture or to find out who shot J.R. are just about gone.

That makes "ER" the last blockbuster, says Robert Thompson, who directs the Center for the Study of Popular Television at Syracuse University.

"From the rise of commercial radio in 1929 to 1989, when cable kicks in, we were in the midst of this astonishing period," Thompson says. "Something happened that never happened in the entire history of humanity: People were paying attention to the same stuff at the same time.

"Neither the Greeks nor the Romans nor the medieval church had that kind of command of people's attention. It crossed all lines--racial, gender, rich and poor, old and young. It's an extraordinary cultural phenomenon. . . . With 'ER's' ratings beginning to collapse, you're seeing the vase of mass culture shattering into a million pieces."

It's hard to work up much intellectual passion for "That Girl" and "Kojak," but in a three-network era these shows did provide a national focus of sorts, Thompson says. And that, too, is the larger meaning of "ER"--that it is the last show with a big enough audience to be a touchstone among strangers, a unifying element for the conversation at the bus stop. Thompson argues that as television audiences fragment--including "ER's"--that social grease is disintegrating.

"You take away the national [TV show] and what's going to happen?" Thompson asks. "We may find that kind of consensus of national culture was more important than we realize."

All of this strikes Lydia Woodward, "ER's" executive producer, as burying the patient prematurely.

With the calm of a master surgeon confronting an appendectomy, Woodward evinces no anxiety about the program's immediate prospects. "We don't in any way expect to do anything different than what we already do. We feel we have a strong show, with an incredibly strong cast."

With six of 22 new episodes already produced, Woodward is still negotiating with Clooney over his reappearance as heartthrob Doug Ross. When last seen, Clooney's character had headed off to Seattle, leaving the volatile Hathaway pregnant and uncertain. His return--presumably for the twins' birth--could have an electrifying effect on the series and the audience, since it would culminate five years of the Ross-Hathaway plot line (Margulies is leaving at the end of this season, too).

"We'd like to have George back for one or two episodes, but it's undetermined at this point," says Woodward. "George has expressed an interest and a willingness to come back. . . . At this point, it's a friendly negotiation between us."

As for the end-of-an-era talk, Woodward--who is herself departing at the end of this season--offers a different diagnosis. "It's an interesting question about the ratings," she says. "We get asked that a lot. My personal feeling is it's hard to get too excited about the Number 1 show on TV dropping in its fifth season. Most TV series don't last this long, let alone hold on to the Number 1 or 2 rung on the ladder."

True, but the rungs are getting lower all the time.

CAPTION: New life for an aging drama? Alan Alda guest-stars on "ER," which starts its sixth season tonight on NBC.