In the deafening aftermath of the silent boos from the Gridiron Club's white-tie crowd Saturday night, President Reagan was asked yesterday morning if he regretted using the line "I think we should keep the grain and export the farmers."

"Yeah, 'cause I didn't get a laugh," he told a press breakfast.

And that was just the beginning. It didn't take long before a growing number of incensed farm state lawmakers and lobbyists were demanding an apology.

"This so-called joke is really a thinly veiled accusation that the agriculture problems today are due to our farm families and that the solution is to get rid of them," said Sen. Tom Harkin (D-Iowa). "We all enjoy a good joke -- farmers, too -- but there is a difference between good humor and sick humor."

Said Bob Mullins, a spokesman for the National Farmers Union: "He's been insensitive to everything else. Why should he apologize for this one?"

Farmers now know "the warm and jovial smile of the president . . . covers up a cold and cynical heart," Rep. Bill Alexander (D-Ark.) told colleagues on the House floor.

House Speaker Thomas P. O'Neill (D-Mass.) said he did not attend the dinner and thus did not hear the "export the farmers" crack until told by reporters.

"He didn't say that," O'Neill boomed in response. "He's too politically smart to say that."

In fact, by the end of the day, the White House was doing some serious backpedaling. Said spokesman Larry Speakes, "The president regrets his intentions have been misinterpreted by some. As we all know, the Gridiron is a night for political harpoons. Barbs are thrown in many directions -- most of them the other night seemed to be coming at the president himself.

"His remarks are in no way indicative of the sympathy and understanding he has for many American farmers who face difficulties," Speakes said.

Meanwhile, those who think they see the pointed pen of Pat Buchanan, White House director of communications, in Reagan's remarks simply don't understand how presidential remarks are written.

For instance, when asked last week if he had anything to do with inserting the line "Make my day" in Reagan's dare to the Hill about a tax hike, Buchanan, under whose administration White House speech writers fall, said, "Everybody else did, too.

"If there was an embryonic move towards increased taxes," Buchanan added, "it knocked it dead."

Yesterday, however, "everybody else" was asking everybody else, "Did he use my joke about . . .?"

Everybody else in the speech writers' office, that is, but Buchanan. His office simply issued a "no comment." Which may be appropriate, since the president maintains final edit on all his speeches.