You watch "ER" every Thursday night, but can you hum its theme song?

How about "Law & Order"? "The Practice"? "Third Rock From the Sun"?Fuggeddaboudit.

It's not short-term memory loss you're suffering from; it's melody loss.

Each show may have an identifiable sound - imagine that snapping rubber-band bass line from "Seinfeld" - but many of today's TV themes often use echoing drums, reverberating bleeps, jazzy riffs and ambient sounds as musical shorthand to set the mood.

"Ten years from now, who's going to remember the theme from `The Drew Carey Show'? `Veronica's Closet'?" asks Jon Burlingame, a music history teacher at the University of Southern California.

Former ABC-TV programming executive Ted Harbert rang the death knell in 1994, saying theme songs were obsolete. Harbert's network, followed by others, began jumping directly into TV shows with a so-called cold start, followed by quick titles and few snippets of music.

"Part of the theory is, hey, we've got 22 minutes to hook the viewer," says Peter Roth, president of Fox Entertainment Group. "We cannot afford to spend 30 or 45 or 60 seconds trying to entice them into a song that in most cases is not memorable. . . . We can't give the competitor an opportunity to grab those eyeballs from us, so we'll give you a five-second `stinger' and let the audience be hooked to the show, as opposed to the theme.

"God help us," Roth adds, "I'm sure this was researched carefully by people with pocket protectors."

The networks argue that it was fine to settle back with a leisurely theme song when there were just three broadcast networks and you had to hoist yourself out of the La-Z-Boy to change the channel. But not today, when viewers have 57 choices and a lightening-fast thumb on the remote.

Which is why it's easier to sing the theme song from a '60s chestnut such as "Gilligan's Island" than "NYPD Blue." Or whistle the opener of "The Andy Griffith Show" than recall the theme of "Just Shoot Me."

"What (the networks) don't understand," says Robert Thompson, director of Syracuse University's Center for the Study of Popular Television, "is that the things that make television most beloved are the things they're systematically eliminating."

In the early days of TV, shows licensed existing music, such as "The William Tell Overture" for "The Lone Ranger," says Burlingame, the author of "TV's Biggest Hits: The Story of Television Themes From Dragnet to Friends."

"Peter Gunn," the 1958 show known for Henry Mancini's jazzy theme, became "the first television series where music took on an importance all its own," Burlingame says.

Through the '60s, theme songs laid down the "back story," or set-up, behind classic half-hour sitcoms such as "Mr. Ed," "Gilligan's Island," "Green Acres," "The Addams Family" and "The Beverly Hillbillies," whose theme was played by country-music greats Lester Flatt and Earl Scruggs. The "Hillbillies" song told "America's story . . . in the pithiest way it could be," Thompson says. "Jed goes out there and pricks the soil - with an errant bullet, no less - and what does he do but head West."

Even before those '60s shows were exhumed by cable, they had been kept alive through their songs, which were the stuff of high school, summer camp and college parodies and tributes.

Post's "Rockford Files" theme kicked off a new direction in the '70s, when he and other composers with rock 'n' roll and jazz backgrounds started using hard-hitting drums, synthesizers and electric guitars. The '80s saw "Miami Vice" use an all-synthesized score, no orchestra needed.

And the '90s? The era of New Age music has brought mood-inducing, not melodic, themes. While themes may be jazzy, pulsing, even sophisticated, many aren't hummable. And they won't last in pop culture, as some older theme songs do.

"The Golden Age is obviously over," says Thompson, the TV academic. "We're in the Dark Ages, the medieval period of TV theme songs."

There are, of course, exceptions to the "hook the viewer in five seconds" rule.

"Searchin' My Soul," the theme from Fox's "Ally McBeal," is from a six-year-old album by blue-eyed soul singer Vonda Shepard.

Like the "Cheers" theme, "Where Everybody Knows Your Name," and the "Friends" opening, "I'll Be There for You," Thompson says, "Searchin' My Soul," is not so much a theme as "a pop standard . . . you expect to hear on mainstream FM radio along with Neil Diamond, Barbra Streisand and all those kinds of hits."

And not every show needs a full theme song for its musical fingerprint. It takes only four low notes - "dum-da-dum-dum" - to summon up "Dragnet." Or the wavering "doo-doo-doo-doo" to conjure the eerie "Twilight Zone."

"It would take 10 minutes to explain what those four or five notes do," Thompson says.