Baby-boomers of a certain age can recall sitting around college dorm rooms in the late 1960's and early '70s, giggling and listening to albums by Firesign Theatre until they'd learned every non sequitor by heart. They still knew the lines, and shouted them out, when the reunited Firesign Theatre performed on Saturday night at the Beacon Theatre as part of its 25th-anniversary tour.

The material dated to 1968 to 1971, and only the occasional drug jokes fell flat. Although most of the group's material has been audio only, its four members - Philip Austin, Peter Bergman, David Ossman and Philip Proctor - can also act. The group staged its sketches with quick-change costumes, hats, wigs, along with an old-fashioned sound-effects setup (squeezing a cornstarch box for steps through snow, popping bubble wrap for a fire). Now and then, there were updates, noting a bumper sticker that said: "Are you co-dependent? I can rescue you!" or envisioning Rudolph Juliani battling an armed, seceding Staten Island.

In its heyday, Firesign Theatre concocted what might be called "stream of comedy": sketches that carried the conventions of radio drama through the free-associative warp of psychedelia. Whereas its English contemporaries, Monty Python, often relied on visual slapstick, Firesign Theatre was pure head trip. Dropping allusions to literature, rock songs, B movies,Eastern mysticism, World War II propaganda and television commercials, Firesign Theatre came up with multi-level scenarios that could veer in any direction on the turn of a pun. It dissolved genre parody into a post-modern whirlpool of information. In Firesign's high-school pep rally, a heckler interrupts the principal by yelling, "What is reality?"

Long before academia discovered Jean Baudrillard, Firesign Theatre defined modern American life as a playground of simulations and decontextualizations, hucksters and disinformation. In "I Think We're All Bozos On This Bus," a romp through a computerized dystopia, the President (Mr. Austin) has become a bromide-dispensing robot at a theme park, destroyed by a renegade programmer (Mr. Proctor). In "Don't Crush That Dwarf, Hand Me The Pliers," an actor (Mr. Ossman) goes channel-surfing and reminiscing, from his youthful "Porgy and Mudhead" movies to his last infomercial.

Firesign Theatre's light touch and allusions within allusions might tax the attention span of audiences used to sitcom punch lines and shock jocks. With their leisurely details, the group's routines can seem as vintage as the 1940's radio dramas it spoofed in ":Nick Danger," although they haven't turned kitchy. But considering how the Firesign Theatre was a generation ago, it would be fascinating to find out, at length, how the group sees the 1990's.