When Americans heed the call to "Write Your Representative," Capitol Hill employes have learned to brace for the consequences.

The mail that members of Congress receive--millions of pieces a year--is only exceeded by the mail they send, $8.1 million in individual letters, district-wide newsletters, questionnaires and "Dear Friend" communications. They keep their staffs, the Government Printing Office and even majority and minority party printers busy churning it out.

And between the incoming and outgoing mail, casework correspondence, press releases and official congressional printing, Congress is drowning in paper, according to several officials on the Hill. Official paperwork generated on the Hill accounts for more than 90,000 tons a year, in one estimate.

Incoming mail "takes hours to open," sighs an aide to Rep. Beverly B. Byron (D-Md.), whose office sometimes needs two or three people and an entire morning just to cope with the mail generated by a particularly hot issue.

The crush of incoming mail can so tie up the flow of correspondence that Rep. Stan Parris (R-Va.) often tries to shorten delivery time for outgoing mail by bypassing the Hill's central postal system.

"We've started taking mail for our constituents and carting it off to the post office in Springfield to be mailed," a Parris aide reported. He said incoming mail sometimes takes three days to get to the office from the time it arrives in the building. He complained that it once took five days to send a letter from Parris' office on one side of the Capitol to a Virginia senator's office on the other side.

Since January, congressional offices have been deluged with correspondence in connection with the proposed repeal of the new interest and dividend withholding law. For a time that controversy helped boost the mail to about 800,000 pieces, more than three times the "normal" 250,000 mail volume.

Overall volume, according to House postmaster Robert V. Rota, who oversees a staff of 118, has climbed steadily from 14.6 million pieces in 1969 to 161 million pieces in 1981. During the Watergate era, which he said was the first real mail campaign to hit the Hill, the House committee considering whether to impeach Richard Nixon received 3 1/2 million pieces of mail a month. And in 1981, President Reagan's budget speech sparked the receipt of 1.2 million letters a day, including post cards and mailgrams.

House members, Rota added, also were inundated with another 800,000 "Save School Lunches" pie plates.

For its part, Congress uses up about 80,000 to 90,000 tons of paper a year through official Government Printing Office work, according to one rough estimate. This is only a small part of the 350,000 tons or $1.2 billion a year worth of printing and duplicating generated by the entire federal government in fiscal 1982.

Casework, an essential constituent service for any member of Congress, requires additional paper work and correspondence.

"The mail remains the cheapest means of communication with constituents," said Rota, who noted that one letter to a member of Congress may generate as many as 10 more in order to handle a constituent problem.

Communications needs have certainly changed since Congress established its own printing operations more than four decades ago, says David R. Ramage, majority printing clerk for the House.

"Back then, they just had mimeograph machines for sending out whip reports and leadership notices," says Ramage, a private printer who serves "at the pleasure" of the Majority Leader and has a staff of 20 to set type and print up the congressional district newsletters, town meeting notices and other printed materials sent out by the Democrats.

His office, located in a federally owned printing bunker on Capitol Hill, is papered with donkey figures and political posters from the campaigns of Lyndon Johnson and Jimmy Carter. He bills the Democrats for his services but won't say what he earns from their business. Next door, surrounded by elephants and other tokens of Republicanism, is Tom Lankford, the minority printing clerk.

House members pay for printing services and other office expenses out of a special office allowance, which averages $122,000 a year.

Senators use the same Senate printer, regardless of party, and are governed by stricter rules about office expenditures. They have, for example, a specific "paper allowance" which they can exceed only by "borrowing" the unused paper stock of another senator--with that senator's written permission.

"All it takes is a letter--they work it out in the cloakroom," said a Hill staffer.

But until that trade-off is approved, says Frank Curtis, the Senate's director of printing services, "I just don't take his work."

He acknowledges, however, that the senators get their material printed and distributed one way or another: "Have I ever cut anybody's water off? Yes, but it doesn't stay off for very long."