More than 307 million letters are mailed from congressional offices every year.

Most of them are as innocuous as the response that is sent from the White House to fans of Amy Carter. A replica of a handwritten note, it reads: "Thank you for writing to me. It's fun living in the White House, and I'm glad you're my friend."

No matter how earnest or anguished the constituent's original, almost all of the replies sent from the Hill are carefully composed form letters. But sometimes the mail simply won't fit the programmed responses and humans, not machines, have to write the answers. That's where the trouble comes in; and in the summer many of those humans are interns.

One of them, Rick Fijolek, was working last summer in the office of Rep. Max Baucus (D-Mont.) when a constituent sent in a lighthearted letter criticizing mandatory participation in the Social Security system.

The reply, which Fijolek wrote and put in the "out" box - and which apparently went directly into the mail said in part:

"The IRS people are very nice. They allow every representative to send in the names of 20 people and then they audit these people. You see, the people at IRS understand that we get a lot of ridiculous letters from our constituents and want to help us get a few of them off our backs, so to speak . . .

"One of my staff saw your letter and figured you were a real crackpot what can I say except that I'm sorry I figured I'd warn you so you could get your files in order."

The constituent didn't find it funny. And now, a year later, Baucus' opponent in the Montana Senate race, Larry Williams, is threatening to use the letter somehow against Baucus in the campaign.

Fijolek, an honors student entering his senior year at Stanford and working this summer for the Democratic National Committee, said that he saw some humor in the constituent's letter and tried some of his own in reply. He said he is "surprised" that the letter, written a year ago, is still a topic of conversation.

Fijolek's problem is hardly unique. Constituents have been offended before - often resulting in a brouhaha.

In 1972, presidential press secretary Jody Powell, when he was working for then Gov. Carter in Georgia, gained some meassure of notoriety for a personal reply to a letter written to his boss by an Augusta women.

She called Carter "a gutless peanut brain." Powell fired back, calling her a "moron" and suggesting that she "take two running jumps and go straight to hell."

Just the human touch a politician likes to do without.

And an aide to the late Alabama Sen. James Allen earlier this year was suspended for writing letters over his own signature that Allen said violated good taste. To a Seattle man, the aide wrote the Washington state was "a melting pot of neurotics, cranks and other individuals with submormal mentalities." A Massachusetts women said the aide called her a "crackpot."

In fact, something like Fijolek's letter may not be unusual at all, says William Hildenbrand, secretary for the minority of the Senate.

"I suspect it happens more often than we know," he says. "So much mail goes out of this place. But constituents probably let it go."

Interns handle mail because, as one said, there are "incredible piles" of it on the Hill. They may write original letters if all the form letters don't apply, or simply out of boredom. In the course of a Washington summer, answering constituent mail can become tedious, perhaps not as glamorous as the intern may have imagined.

"Some interns will come in and expect the first week to sit down at lunch with the congressman and be an adviser, and to write legislation the second week," says Todd Feigenbaum, internship supervisor at the Washington Center for Learning Alternatives, which places and advises interns. "We quickly try to bring them down to reality with an orientation procedure."

But controversial letters have also come from seasoned former politicians Ohio Sen. Stephen M. Young, who left the Senate in 1970, is regarded as the champion plain-spoken letter-writer.

Among his replies to constituents:

"Dear Sir,

"You are a crackpot.

"In addition, you are a liar.

"Furthermore, you manifest gargantuan ignorance of my voting record,"

And:

"You are the south end of a horse going north . . ."

And:

"Don't give me any more of your unsolicited advice." I know it costs nothing, but that's exactly what it's worth."

And:

"Buster, you will be wasting postage unless you remove me from your mailing list."

Though it took a campaign opponent to keep Fijolek's letter in the news, it's a matter of record that many people take their mail from Washington seriously. And kind of mail.

Last year there was a widespread outcry when the White House tried to change its policy on presidential birthday greetings (programmed ones, of course). Greetings used to go to any citizen ago 80 or older, on request. The White House tried to up the age to 100. It quickly backed off. "We did not correctly calculate the widespread knowledge of this policy," a White House official said. The safe response was to send the safe letter.

For Washington politicans, the "safe" response is the dull one sent out by machine.But sometimes even the machines fail. One man who returned his war medals in protest to the White House received a stock response letter thanking him for his "thoughtfulness."

A woman who wrote to the Carter White House about the naming of Jack Tanner as a federal judge received an answer from a White House letter by writing machine thanking her for her recommendation of "Warren D. Reibe for a position in this administration."

It turned out that Reibe had been recommended for a position years ago in the OMB. But he chose to stay in Cleveland.