The last time R.E.M. performed in Washington--at the 1998 Tibetan Freedom Concert at RFK Stadium--it was a momentous occasion on many levels.

R.E.M.'s set, the band's first public performance since the departure of founding member and drummer Bill Berry, had been scheduled for Saturday, but was canceled after a massive thunderstorm put an end to the day's performances. When R.E.M. slipped into the Sunday schedule with a shortened set, the band chose to introduce three new supporting musicians and four new songs from an album it had just spent six months recording. Slow, deliberate, challenging songs that the 65,000 fans in attendance had never heard before.

"The original plan was to do seven brand-new songs, but we chickened out a little bit," says guitarist Peter Buck, who will lead R.E.M. into Merriweather Post Pavilion on Friday. Buck concedes there was a high level of incomprehension in terms of audience reaction to those new songs at RFK. Or maybe concertgoers didn't know quite what to make of lead singer Michael Stipe, who performed that day in a flowing saffron sarong.

"What everyone assumed we would do is just go out there and play all our hits, all in a row, like every other band did basically, and then it's like a celebration," Buck explains. "But that's just too easy, it's cheating. Perhaps it was stupid that the first thing we did with Bill gone and a whole new lineup of people was not be an R.E.M. cover band. . . . It wasn't the best performance we ever did, but I like the fact that we opened with something so incomprehensible ["Airportman"] it killed the crowd."

R.E.M.'s subsequent album, "Up," was released that fall, and it confirmed that changes weren't limited to personnel. Having evolved from jangly alt-rock pioneers in the early '80s to reluctant pop superstars in the early '90s with "Automatic for the People"--that album sold 12 million copies worldwide--R.E.M. seemed to challenge itself all over again in the last half of the decade, beginning with the sludgy sonic throttle of 1994's "Monster" album.

It was during the subsequent "Monster" world tour that Berry almost died from a brain aneurysm. An Athens, Ga., homeboy, Berry had always been the most reluctant about committing to year-long tours. When R.E.M. got together in 1997 to talk about a new album, he informed Stipe, Buck and bassist Mike Mills that, after 17 years, he was leaving the band.

There was shock, disappointment, even brief discussion about the band calling it quits as well. Buck says Berry's decision didn't come as a complete surprise.

"In retrospect, I can see that Bill had probably already made up his mind to split," Buck says. "I thought it might be personal things as opposed to being fed up with the band, but he just wasn't committed anymore. Generally, even if he didn't bring in songs, he would be working on ours. But this time, he spent a lot of time looking at the ocean and stuff.

"I was sad to lose him as a friend, but he hadn't been interested in doing it for a while and it was just so painful for us to get him to do anything," Buck adds. "Bill wasn't being a bad person or anything, but he just didn't want to do it and we did want to do it."

Berry's departure coincided with R.E.M.'s subtle shift to drum machines, sequencers and tape loops, a sonic experiment necessitated by geographic distance after Buck, who writes much of the music, moved from the band's longtime home in Athens to Seattle.

"It was a process of us being far apart, and me particularly," says Buck. "After five or six years of just traveling around and actually setting up in the studio and getting kind of enamored of working by myself, I had a lot of finished songs that had drum machines. And everyone thought, well, that sounds really good."

Stipe, who writes lyrics to existing tracks, explained in the Irish music paper Hot Press that in the past, R.E.M. would often record multiple tracks but that in the final mixes "guitar and drums would take the front seat and everything else would sound interesting but indistinct, occurring somewhere underneath. That underneath stuff now becomes the stuff on top."

Recording "Up" had a certain element of foreboding, Buck admits, but it also represented an unusual opportunity.

"We were redefining ourselves as a band and I guess, therefore, a little bit as people," says Buck. "But going in, it felt really positive and finishing it was positive, because we knew that we could reinvent ourselves, be a different group, make a record that didn't sound like anything we'd ever done, a record that we felt really strongly about."

In the studio, the remaining members of R.E.M. further experimented by switching instruments--guitarist Buck played a lot of bass and bassist Mike Mills shifted to keyboard--and incorporating new musicians like former Screaming Trees drummer Barrett Martin, Beck percussionist Joey Waronker and keyboardist Scott McCaughey (who plays with Buck in Tuatara and the Minus 5). On tour, they're joined by former Posie Ken Stringfellow on bass, keyboard and guitar.

According to Buck, R.E.M. initially planned not to tour behind "Up." When the band finally decided to go back on the road--it recently finished a European tour and embarked on a stateside swing last month--R.E.M. briefly considered working with prerecorded backing tracks to reflect the newer, increasingly complex sounds of the new album.

"We tried it a little bit and I hate playing to tapes," says Buck. "It takes away all the elements you like live, particularly spontaneity. You can't just decide to play a song because it's not cued up; you can't speed up or slow down, it's the same tempo every night. So we tried to figure out how to make the record sound live."

Meanwhile, radio hasn't quite figured out how to make the new R.E.M. sound like the familiar R.E.M. That's true of both commercial radio--which embraced FM-friendly fare from 1987's breakthrough hit, "The One I Love," to "Losing My Religion" and "Everybody Hurts"--and college radio, the band's best friend during its first decade.

"The place where we are in our lives and in our careers, it's not been particularly important what radio format 'Up' fits into or who hears it," Buck insists. "It's more important to please ourselves and figure out who our audience is going to be in the future. We might be in a place where maintaining a huge audience is not so important--and it's never been that important to me."

It might be important to Warner Bros., though, particularly since the band signed an $80 million record deal five years ago in the wake of the multi-platinum success of "Automatic for the People." The follow-ups, 1994's "Monster" and 1996's "New Adventures In Hi-Fi" (written on tour and recorded during sound checks and concerts), sold only 6 million and 3 million, respectively, and "Up" has sold 3 million. Ever since 1992, when Soundscan started tracking and publicizing weekly sales figures, such numbers have become an albatross for bands in much the way that weekly box office grosses have for filmmakers. It's fine when you're on top, but disappointing reports on commercial performance often obscure creative accomplishment.

"I'd feel bad if U2 and Pearl Jam and Smashing Pumpkins were selling 10 million records and we weren't, but everyone kind of hit the same little skid," says Buck. "Maybe it's just not our turn. And that's fine. We've still got a lot of work to do and we feel strongly about it. Oddly enough, we had a really big hit in Europe, though 'Up' doesn't feel like a particularly European record. In France, they asked, 'How do you account for your huge comeback?' "

To borrow a concept from Chumbawamba, to fall down is to get up again, a theme that's actually addressed a number of times on "Up." In songs like "Falls to Climb," "Parakeet," "Airportman," "Daysleeper" and "Hope," characters fall down but they get up again. Or they find themselves in awkward, disastrous situations that they eventually resolve. And though Stipe's lyrics remain as opaque as ever, you can actually read them and then try to figure out what the songs mean: After 13 albums, R.E.M. included its first-ever printed lyrics in the CD booklet.

And as the band embarks on its first U.S. tour in four years, the band is also at work on a follow-up. Buck, who travels with a portable eight-track studio, says there's already an album's worth of music, though only one actual song so far.

And that's in addition to the score R.E.M. has crafted for "Man on the Moon," the upcoming Jim Carrey biofilm about tormented comedian Andy Kaufman. Kaufman, of course, was immortalized by the R.E.M. song from "Automatic for the People," and its success helped bring him back into the public eye.

According to Buck, besides a reprise of the title song and a new song R.E.M. is performing in concert, "The Great Beyond," the score is partly orchestral, partly for small jazz ensemble, with some incidental music as well. The writing process, he points out, is almost the complete opposite of the way it is on normal R.E.M. projects, where Stipe comes in with the lyrics after the music is completed.

"Here, we'd put the movie on and sit down and write, with a list of cues," says Buck. "We'd sit at the keyboard and kind of pike along until something was occurring that we liked and then we'd work on it. Quite often I'd come up with structural things, chord changes, and Michael would flesh out the melodies, because he's better at that than I am.

"It was pretty fascinating, partly because we're quite autonomous when we work by ourselves. But in this instance, we'd send something out [to the director and producer] and they'd come back and say 'We like this, we don't like this, do this again . . .'

"It was a learning process."