Is Ronald Reagan tiring of his famous anecdotes? Has Don Regan told him to knock off the funny stuff and get down to business? Is the president, perhaps, beginning to worry that he may go down in history as the Henny Youngman of Pennsylvania Avenue?

Whatever the reason, at his Tuesday night press conference he delivered a punch line but left out the story.

On the subject of tax increases he had approved, Reagan argued that he'd been promised "$3 in spending cuts for every dollar of increased revenue.

"I never got the $3," he said. "So I'm like the fellow in the story, I'm still yelling, pay the $2 -- only three in my case."

What story?

An informal poll of reporters at the news conference drew a blank -- no one got the reference. The White House press office was also puzzled, and dispatched Kathleen Osborne, the president's personal secretary, to the Oval Office to query the source.

What Reagan had in mind, it seems, was a variation on an ancient joke. White House spokesman Pete Roussel said yesterday that it goes something like this:

A man is charged with a traffic violation, or with spitting in a subway, or whatever, and is fined $2. He hires an attorney, who proceeds aggressively to challenge the charges in court. The louder the attorney protests, the higher the judge raises the fine.

Finally, in frustration, the defendant yells to his attorney: "Pay the $2!! Pay the $2!!!"

Dan Brenner, an attorney at the Federal Communications Commission and a local stand-up comedian, said the story is an old vaudeville routine. "The way it goes is that the guy is saying from the beginning, "Pay the $2," and the attorney is saying 'No, no. We'll fight it.' Soon the guy loses his house, his clothes, his car, and the skit ends with him walking around the stage in a barrel."

"Yeah, yeah, yeah, I remember the story. PAY THE $2! PAY THE $2!!!" confirmed the real Henny Youngman, when reached in Florida where he was performing with Milton Berle. "You see, the president was in show business, so he remembers all these stories. It's a hundred years old. I did the skit once. Everyone did it once.

"This guy gets a ticket, and his lawyer says, 'Don't pay it, I'll handle it.' So he keeps telling the lawyer, 'Pay the $2! Pay the $2!' So the guy ends up getting thrown in jail and losing all his money. And he says 'Will ya pay the $2?' It's funny. The point is, don't make a federal case . . . just pay the $2."

Milton Berle could not be reached for further clarification.