Play that jazz.

Play that rag,

Grab your baby, and dance that shag.

A swell time's always in the bag

At a Saturday night to-do.

Sing those lyrics, with the bouncy tune written for them, to any connoisseur of old-time American pop music and ask him to guess the approximate date of composition. If he says the mid-1920s, the Jazz Age or the Prohibition era, you should give him credit, even though his answer would be technically incorrect.

Nobody could be expected to know that "A Saturday Night To-do" is brand-new. Or that it and four other songs of similar vintage are part of "The Great Gatsby," an opera scheduled to premiere tomorrow at the Metropolitan Opera for broadcast (locally on WGMS) on New Year's Day.

Based, obviously, on F. Scott Fitzgerald's classic novel, "The Great Gatsby" was composed by John Harbison, who also wrote most of the opera's words. The ones he didn't write are the lyrics of the pseudo-vintage songs scattered through the tragic opera: fox trots ("Dreaming of You" and "Kind of in Love With You") and ballads ("I'm Doin' Fine" and "Cool"), coming out of an onstage radio or sung by a band vocalist at one of insecure, nouveau-riche Jay Gatsby's "wonderful, intimate, large parties" on his pretentious Long Island estate.

These songs clearly help establish the show's atmosphere, like the scenery and costumes and automobile styles, but they are more than a kind of musical wallpaper. Thematically linked to the characters and stage action, the "Gatsby" lyrics are the work of Murray Horwitz, National Public Radio's vice president of cultural programming and a jack-of-all-trades (on second thought, make that Renaissance man) of entertainment.

Horwitz, who began his career with three years as a clown for Ringling Bros., has acted onstage (for example, at the New York Shakespeare Festival), on television (in "Kojak") and on film ("Night of the Juggler"). He was a co-author and lyricist for the hit musical "Ain't Misbehavin'," performs on NPR as an anonymous voice in comic sketches, and each year reads Hanukah stories on NPR with Susan Stamberg.

Horwitz said he and Harbison became friends at first sight when they met in the early development stages of "The Great Gatsby" at the Metropolitan Opera.

"He liked me and I liked him, and we quickly reached agreement on how to create the songs. He wrote the tunes first, and I fitted the words to them," Horwitz said in an interview. "His tunes sound like the real thing, and that made them easier to work with. He didn't produce a slavish imitation of 1920s music, and he didn't produce parodies.

"We tried to write the kind of songs that would have meant something to the characters, Tom and Daisy and Nick and Jay. We all have a special affection for our generation's oldies, whatever they may be: the songs that we loved, that spoke to us when we were young. What we tried to create would be the fictional oldies for these fictional characters."

Songs of the 1920s are clearly unlike those of the 1930s or such earlier songs as "After the Ball" and "When I Grow Too Old to Dream," and Horwitz's 1920s lyrics show a clear understanding of what makes them different.

"Songwriters of that time had one foot in ragtime and one foot in the more innocent, old-fashioned style of George M. Cohan and other songwriters of the World War I era," he explains. "The key to what they were doing lies largely in the rhythms they were following. The revolution that was made by Louis Armstrong, King Oliver and Jelly Roll Morton was mostly a rhythmic revolution."

About fitting words to tunes that were already written, he said: "Lyricists get ambivalent. You like to be unfettered, but, as Duke Ellington said, 'limitations are wonderful things.' Lyricists of the '20s were writing to fit the music's syncopation. Writing lyrics for 'When You and I Were Young, Maggie' is very different from writing lyrics to a Charleston."

In all, Harbison and Horwitz wrote 14 songs, five of which are sung and all of which are played in the opera at one point or another (for example, a tango series is played by the onstage band right after "Dreaming of You"). All 14 are being published by Schirmer in a collection titled "The Gatsby Songs," and vocalists of many styles (country, cabaret, rock) have shown interest in them.

"We tried to write songs that could become standards, the way Gershwin and Cole Porter produced standards," Horwitz said. "It may be presumptuous, but it is my sincere hope that some of these songs will have an afterlife."