When it was first broadcast in December 1965, no one was quite sure what to make of A Charlie Brown Christmas, the first Peanuts tale ever shown on television. Untrained children did the characters’ voices, there was no laugh track and it had an overtly religious theme, highlighted by Linus’ recitation of the Nativity story from the Gospel of Luke. Network executives were certain it would flop.

Then there was the soundtrack. Producer Lee Mendelson invited a pianist named Vince Guaraldi to compose the music, opting for grown-up jazz instead of the exaggerated effects of most cartoons. Guaraldi’s high-spirited “Linus and Lucy” theme has become almost as recognizable as the Peanuts kids themselves.

“The music was absolutely essential to its longevity,” says Mendelson, who has been the executive producer of every Peanuts TV project since the beginning. “It didn’t catch on right away. It was all serendipity.”

(I Want a Dog For Christmas, Charlie Brown will be rerun tomorrow at 8 p.m. on channels 5 and 6)

After four decades, Guaraldi’s subtly infectious soundtrack has just gone double platinum, with sales of 2 million. It was recently released in an expanded and remastered version with a brighter sound that gives fresh clarity to Guaraldi’s inspired work. (The sidemen have finally received proper credit as well.)

Perfectly pitched to the moods of the show, the music underscores the humor, innocence and wisdom residing within Charlie Brown and the rest of cartoonist Charles Schulz’s world. Virtually every American has heard the soundtrack, but the story behind it — and the career of the late jazzman Guaraldi — remains largely unknown.

IN 1963, Lee Mendelson was a young San Francisco filmmaker working on a documentary about Schulz, whose Peanuts cartoon strip was fast becoming a national craze. He needed music for a two-minute animated segment of his film. Driving across the Golden Gate Bridge, he heard a catchy jazz tune on the radio called “Cast Your Fate to the Wind,” which was written and performed by Guaraldi, who also lived in the Bay Area.

Guaraldi, then 35, was a journeyman jazz pianist who had toured in the 1950s with Woody Herman’s Thundering Herd and with vibraphonist Cal Tjader and trombonist Bill Harris. He had made a few recordings with his own groups and was one of the first American musicians to discover the swaying rhythms of the Brazilian bossa nova.

In 1962, Guaraldi released Jazz Impressions of Black Orpheus, interpreting music from the 1959 Brazilian film. To fill out a short album, he wrote a tune that was packaged as the B-side of a single. (About the same time, he grew a handlebar mustache, which became his signature look.) That throwaway tune was “Cast Your Fate,” which caught on with listeners and went on to sell 500,000 copies. It reached No. 22 on the pop charts — one of the last instrumental jazz tunes to be a crossover hit — and earned Guaraldi a Grammy Award in 1963 for best original jazz composition.

“Cast Your Fate” had the qualities Mendelson was looking for — “both adult and childlike” — so he asked Guaraldi to write something for his Schulz documentary. Within two weeks, Guaraldi called back.

“He asked if he could play something over the phone,” Mendelson recalls. “I told him I didn’t want to hear it over the phone. He said if he didn’t play it, he might forget it. He played the ‘Linus and Lucy’ theme. It was so perfect. Somehow, in my soul, I knew this was going to have a deep impact on what we were going to do.”

As it turned out, Mendelson’s documentary never made it to television. (It’s now on DVD.) But Guaraldi recorded an album originally called Jazz Impressions of a Boy Named Charlie Brown, which came out in 1964 and, with its swinging whimsy, may be his true Peanuts masterpiece.

In April 1965, CBS decided to do a Peanuts special for Christmas. Mendelson and Schulz worked out the story line in one day, and Guaraldi was brought in for the music.

He reprised the “Linus and Lucy” theme, rearranged some familiar holiday tunes and added new compositions (“Christmas Time Is Here,” the buoyant jazz waltz “Skating” and the infectious dance number “Christmas Is Coming”). Mendelson wrote the lyrics for “Christmas Time Is Here” on the back of an envelope in 15 minutes.

Guaraldi was a short man, about 5 feet 6, with thick glasses and a sly, understated sense of humor. He had been a regular reader of Peanuts since its debut in the early 1950s, and it’s fair to say he took the plight of the beleaguered Charlie Brown to heart. His compositions contained a palpable depth of feeling.

“I want to write standards, not just hits,” he once said.

His accessible style, with its deceptively simple melodies, shifting rhythms and graceful improvised passages, proved ideal for the film’s wistful tone.

“With the music, you’re able to crawl inside the cartoon character,” recalls David Guaraldi, who runs a Web site devoted to his father’s career.

The musicians in Guaraldi’s trio — bassist Fred Marshall and drummer Jerry Granelli — sensed that something special was coming together, but no one knew what to expect from the overall project. CBS was lukewarm, and even the production team had doubts.

“We thought we ruined Charlie Brown,” says Mendelson. “We thought it wasn’t a good show.”

An early screening for CBS execs was greeted with silence.

“Then one of the animators, who had had a few drinks, stood up in the back and said, ‘You guys are crazy,’ ” Mendelson recounts. ” ‘This is going to be around for a hundred years.’ ”

WHEN THE special aired on Dec. 9, 1965, it drew a 50 share, meaning half the country was watching. Critics loved it, and the Peanuts franchise was launched. Four decades later, A Charlie Brown Christmas still scores high in the ratings and has entered our culture as the television-age equivalent of Dickens’ A Christmas Carol. (The special ran this year on Nov. 28 on ABC.)