George Carlin, 71, the much-honored American stand-up comedian whose long career was distinguished by pointed social commentary that placed him on the cultural cutting edge, died last night in Santa Monica, Calif.

He had long struggled with health problems and a heart condition dating to the 1970s, and, according to Associated Press and other reports, he had checked into the hospital yesterday after experiencing chest pain.

Carlin's comedy career spanned a half-century, starting with years as a disc jockey in the 1950s and culminating with his selection last week by the John F. Kennedy Center for the Performing Arts to receive this year's Mark Twain Prize for American Humor, a lifetime achievement award.

Over that time, he evolved from the more straight-laced member of a comedy duo formed with Jack Burns into a social satirist whose routines deliberately tweaked the social and cultural edge -- mocking religion, sexual prudery and American society.

In an interview last year with The Washington Post, Carlin said that as his career evolved, "I found myself in a divorce from my species and culture" and changed his focus from mainstream nightclub stand-up to the biting, observational humor that made him famous. He teased the shallowness of radio programming in skits about the fictional station WINO, and the shallowness of television personalities with his "Hippy Dippy Weatherman."

Those lighthearted parodies gave way, however, to a more serious exploration of language, social mores and particularly "religious superstition."

It was a career that both tracked the changes underway in the 1960s and 1970s, and helped mold them. His 1972 album "Class Clown" included the signature "Seven Words You Can Never Say on Television" -- an expletive-filled bit that led the Supreme Court to clarify rules for what could be said on radio and television and when.

In the spirit of 1950s comic Lenny Bruce, whose monologues wore down the significance of offensive language through repetition, Carlin's routine took a list of words which, ostensibly, could not be said on television, and worked them into contexts that ranged from the Bible to a housewife in the kitchen.

Carlin was arrested for performing the monologue live in Wisconsin, though the charges were dropped by a judge who found the material indecent but protected under the First Amendment.

However when a New York radio station aired a similar bit by the comic, a complaint to the Federal Communications Commission ended with a Supreme Court ruling which, in fact, upheld restrictions on language that was "patently offensive," not just obviously obscene.

While the media landscape has changed markedly since that 1978 ruling -- the content available on cable television and the Internet makes Carlin's monologue seem almost quaint -- the controversy secured his reputation as a social critic.

"In his lengthy career as a comedian, writer and actor, George Carlin has not only made us laugh, but he makes us think," Kennedy Center chairman Stephen A. Schwarzman said in announcing Carlin as this year's Twain Prize recipient.

Though an iconoclast, Carlin was embraced by the culture he mined for his material. He made dozens of appearances on the popular television shows hosted by Johnny Carson and Ed Sullivan. Mainstream outlets curbed their interest in him, however, as his material grew racier and his pony-tailed appearance more offbeat.

The popularity of his albums earned him four Grammy Awards -- including his first, for "FM & AM," recorded in 1971 at Washington's old Cellar Door club. He hosted the debut episode of Saturday Night Live in 1975, an appearance for which, he notes on his Web site, he was "loaded on cocaine all week long" -- a reference to substance abuse problems that dogged him throughout his life. Carlin also appeared in numerous movies and television shows, voiced animated characters in children's shows, and enjoyed a steady stream of appearances on cable television, where he was free to say what he wished.

Born in Manhattan in 1937, Carlin grew up in Morningside Heights, and in a Washington Post interview last year traced his interest in comedy to an imitation of sultry film star Mae West that he would perform as a child for his mother's friends.

"I got the attention of the adults and I got their approval," Carlin said.

He dropped out of high school and joined the Air Force in 1954. He began his radio career at age 18 working at station in Louisiana near where he was stationed, then moved to Boston after his discharge from the service to begin his career in earnest.

He once summed up his approach: "I think it is the duty of the comedian to find out where the line is drawn and cross it deliberately."

His first wife, Brenda, died in 1997. They had a daughter, Kelly. A second wife, Sally Wade, survives him.