For nearly 100 years, as the government printing press spewed out official documents, federal workers have dutifully packed them in boxes and sent them to libraries around the world.

In return, as part of the Brussels Convention of 1886, the U.S. Library of Congress has been receiving government publications from about 30 other countries.

The program was working well seemingly without a hitch, until this year, when Sen. Jim Sasser (D-Tenn.) made a discovery that turned his eyes saucer-size.

Sasser learned that the shipments of U.S. documents included military manuals that he considered highly sensitive, were being sent routinely to the Soviet Union, Cuba and Iran, among others.

The more Sasser looked, the more "amazed and dismayed," to use his words he became. U.S. intelligence assessments, CIA maps and atlases and foreign policy position documents were among the publications going abroad.

One that bothered him most was an Army technical manual outlining uses and operation of the Lance surface-to-surface missile system deployed here and in Europe.

Apparently none of the government-printed matter being sent abroad is classified, and apparently it all is shipped with the approval of the agency which originated the document.

Sasser said his inquiries to the Library of Congress about who approved sending the Lance manuals to foreign countries brought the answer that, "Oh, no one. Everything is shipped automatically unless someone objects."

The Tennessee senator also complained that most of the documents coming here from participating countries seemed to be innocuous and of far less sensitivity than items mailed from Washington.

The fuss that Sasser stirred up has sent waves all the way to Moscow, where the press has criticized him, and touched off a legislative reaction that could endanger U.S. compliance with the 1886 convention.

Following Sasser's lead, Rep. Silvio O. Conte (R-Mass.) won House approval last month of an amendment banning use of federal money for book exchanges with the Soviet Union, Cuba and Iran.

Sasser didn't mean for Congress to go that far, for the exchange of less sensitive government publications on a broad range of subjects is considered important by scholars and technicians.

The program came to congressional attention earlier this year when the House passed a bill transferring book-exchange authority from the Smithsonian Institution to the Government Printing Office.

The change was viewed as a house-keeping improvement since the Smithsonian was contracting with GPO to mail the documents anyway. Under the change, the Library of Congress would become the chief administrator.

But the change also meant that the program, for the first time, would show up in the budget of Congress, rather than the executive branch. Sasser, chairman of legislative appropriations and well-known for his sensitivity to rising congressional costs, took a closer look.

He was able, meanwhile, to bottle up the House-passed bill, on the ground that the program ought to stay in the executive branch, but be more closely monitored.

Library of Congress officials, who, as employes of the House and Senate, are reluctant to take a swipe at the boss, are privately bitter about the legislative maneuvering.

One library official, noting that none of the shipped documents is classified, pointed out almost everything that goes abroad also goes automatically to scores of U.S. libraries.

"It's up to the individual agencies to decide if a document is too sensitive to be distributed," he said. "If the concern is the Russians, I'm sure they know more about our Army than a manual will tell them.

"If we don't send it, all they have to do is go to one of the libraries in this country. Or they can go to the National Technical Information Service and buy many of these same things. If we stop shipments, they probably will retaliate."

Sasser and Conte have not proposed that the U.S. libraries not be given the same documents that now go abroad. "If the Russians want this stuff, make them work for it," said an aide to Sasser. "It's dumb for us to send it, postpaid, to the Lenin State Library in Moscow."

Meanwhile, Sasser reported, U.S. defense and intelligence agencies have indicated to him that they will begin a closer review of documents approved for shipment abroad.

The document exchange program costs the United States about $1.1 million for printing and mailing 20,441 publications to about 30 countries in fiscal 1979.

In return for the 1.6 million copies sent out, the United States received 275,000 documents from exchange participants abroad.