Some of the most important men in America, including President Reagan and Vice President Bush, turned up at Saturday night's Alfalfa Club dinner to tell a few jokes, have a few drinks and try to keep it all a secret.

As with all secret societies, there were some ground rules. For one, Reagan was handed a note and said he'd been asked to ask the audience to remain after "the most powerful man in the world leaves." He looked at Don Regan and asked, "Gee, Don, do you think there'll be time for me to leave after you've made your exit?"

He also apologized to the audience, saying, "I would have been here earlier, but with Gramm-Rudman my limousine only made it to the Hay-Adams."

These power brokers, no strangers to public speaking and whose ranks include Walter Mondale, Pat Buchanan and Secretary of State George Shultz, drew upon a reserve of nondescript adjectives to describe the annual evening of merriment and brotherhood: "Wonderful." "Marvelous." "Best ever." "Fun."

If this is the age of information, the Alfalfans certainly don't know it. Ask an Alfalfan to repeat a joke he heard, and he pleads amnesia: "Gee, I don't remember . . . I've got a punch line, but I've forgotten the joke . . . I don't have a good memory for jokes."

Ask an Alfalfan his name and, turning to his friends, he asks, "I don't know, who am I?"

Ask an Alfalfan where the women are, and he rolls over and plays dead: "Oh, aren't they allowed?" asked Walter Mondale.

But Alfalfans know that the last Saturday of every January is when they can let their hair down. Off limits to women, off limits to the press, 650 club members and their guests spend the evening dining in the Capital Hilton's Presidential Ballroom on the customary lobster and filet mignon and painting a fresh coat of humor and foolishness on the old boys' network.

The 220-member network includes eminent representatives from political, military, financial, art and medical circles: John Glenn, David Lloyd Kreeger, Patrick Hayes, David Rockefeller, Charles H. Percy, Sam Nunn, Donald Regan, Alan Simpson, William C. Westmoreland, Edward Bennett Williams, J. Willard Marriott Jr.

Membership in the Alfalfa Club is for life, and age was a topic with the president, as it often is. Of Halley's Comet, he said, "I don't think I'll go out and watch it. I'll catch it the next time around."

The club, which began in 1913, is named for the plant that yields to no obstacle in its search for moisture. Its members pay modest dues for an annual evening of wine-moistened fraternization and the dubious honor of belonging to an organization that in the words of Sen. John H. Chafee "has no designated worthy purpose." In fact, freely translated, the motto ("Bis Dat Qui Cito Dat") becomes "He is double blessed who picks up the check first." And, freely translated, picking up the check, for an Alfalfan, means bringing three (preferably distinguished) guests along for the fun.

Not new to the revelry but newly inducted as members were Duane B. Adams, president of Acacia Mutual Life Insurance; Dwayne O. Andreas, chairman of Archer Daniels Midland Co.; Secretary of the Treasury James A. Baker III; former transportation secretary William T. Coleman Jr.; and Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff. They too donned the red, white and blue ribbon from which hangs the official Alfalfa medallion: a flaming torch bordered by two sprigs of alfalfa with the words "Alfalfa Lux Est ("Alfalfa is light") embossed underneath.

There have been a few power shifts in the Alfalfa inner circle: Fritz Hollings has replaced George Bush as club president; Sen. John Warner assumed the role of vice president; and Robert Strauss became the Alfalfa Party's nominee for president of the United States.

By all reports, Strauss' 20-minute acceptance speech launched a campaign of humor with these one-liners:

*The great philosopher Mae West once said, "Given the choice between two evils, I choose the one I haven't tried."

*If you want a loyal friend in this town, you better get yourself a dog.

*If a mother asks if her son, who has a $35,000-a-year government job, can lead a good, Christian life in Washington, Strauss replies that that's about all he could do on $35,000.

*Strauss lives so far out in Texas that they think Hanukah is a duck call.

What will Strauss do for the Alfalfans if he takes their party to victory? Strauss won't tell. He doesn't believe in revealing his platform.

While Strauss received unanimous support as a political candidate, as a humorist he had some serious competition. Supreme Court Justice Lewis Powell remarked that President Reagan was particularly funny, "always good with one-liners."

Edmund Muskie, exiting with new Alfalfa President Hollings, pointed to his club brother and said, "He's the funny man. He's the funny man. South Carolina's the main event tonight." A couple of examples:

*"I found in my race for president, when E.F. Hollings talks, nobody listens. I knocked on one door, and a lady came and said, 'Who are you?' and I said, 'Fritz Hollings,' and she thought it was a German trucking company."

*Of Bush he said, "George has met more world leaders than perhaps anyone in history. His problem is, before he gets there, they're usually dead."

*Of the balanced-budget act that often doesn't bear his name, "How is it that the media always leaves off the name Hollings? I say, I have more friends in the media than Gramm and Rudman."

Alexander Haig, with a smile stretching from one ruddy cheek to the other, announced that the "naughtiness exceeds itself every year." Post-party naughtiness was not inhibited by the cold, damp night. While waiting for his limousine to slog through the congestion of 16th Street, one boisterous guest inspired hearty laughter with the remark that "Gloria Steinem was the funniest person there. She was fabulous. Just excellent."

Steinem, of course, wasn't there.

The members weren't the only source of entertainment. The Marine Band was called in to play "Hail to the Chief" when the president entered the ballroom around 7:20. The Marine Drum and Bugle Corps, the Sidney Orchestra and two opera singers rounded out the evening -- though by the time the opera singers made their appearance sometime around 10 p.m., many Alfalfans, including the president, had already left.

"Most banquets are self-congratulatory," said Knight Kiplinger, a guest of his father, Austin Kiplinger. "This one is different because it is a self-roast of all the members. That makes it unusual in Washington where people take themselves too seriously."