WHEN Uncle Sam speaks, he says it on a poster:

"Loose Lips Might Sink Ships."

"Air Mail Is Socially Correct."

"Eat the Carp!"

Bringing together 200 years of government messages, the National Archives presents "Uncle Sam Speaks: Broadsides and Posters," a lively look, with all the visual impact that posters intend.

The earliest version of the poster, the broadside, was given its name from the fact that it was printed on only one side of the paper. While they appear at first no more exciting than interoffice memos, what messages they circulated. Here's one King George III sent to the colonial governors, asking for help in suppressing rebellion.

Later broadsides include leaflets dropped on Germany and on occupied France during World War II, urging surrender or warning of an impending bombing. And Civlian Exclusion Order No. 5 regarding the relocation of Japanese-Americans.

And a funny little broadside from the Department of Commerce, with the notation "Please Post Conspicuously," asks Americans to "Catch the carp; buy the carp; cook the carp properly and eat it. Eat the roe; can the roe. Make carp jelly. Can the fish. Smoke it, too."

Early posters here recruit for the Confederacy: "To arms! To arms! Brave men of the west! Drive back the insolent invaders who insult you by their presence on your soil." Or offer a $2,000 reward for Butch Cassidy and the Sundance Kid.

After lithography came to full flower in the 1880s and 1890s, the image crowded out the words and the poster became the flashy medium we know today. Here, a very homely kid touts root beer in a non-government-issue poster: "The Great Health Drink" came to the Archives as a record in a Hires Root Beer trademark dispute and adds the flavor of the times to the show.

It took World War I to turn the U.S. government on to the poster's possibilities for recruitment and inspiration. "Treat 'Em Rough! Join the Tanks!" snarls one poster with an angry black cat leaping through a fiery sky. "Beat Back the Hun with Liberty Bonds," shouts another.

Between World Wars, the federal government found it could still use the poster to persuade. The Post Office politely suggested the use of air mail. The Agriculture Department took the opportunity to educate the population on its standards -- depicting egg candling and grades of beef.

While today's Army recruitment message is "Be all that you can be," in 1925, the Navy offered R&R: "Why freeze? Take a trip to sunny California at Uncle Sam's expense. Visit Honolulu, the Philippines, Japan and China."

World War I posters cheered them on. But in World War II, the message changed to grim realism. "A careless word . . . A needless loss" is the caption for a poster that shows a GI's body washed up on a beach.

To make the message even stronger, Uncle Sam enlisted artists in the war effort. Ben Shahn's moving poster, "We French Workers Warn You . . . Defeat Means Slavery, Starvation, Death," shows desperate-faced men holding rough, empty hands in the air.

But these posters, says curator Nancy Allyn, "are not really art objects. They were meant to be put up and ripped down," in factories and railroad stations.

More recent incarnations of the poster are collectible, such as a series the Park Service issued to celebrate the Bicentennial. "Now, they are meant to be beautiful and decorative, rather than give a message," says Allyn.

UNCLE SAM SPEAKS: BROADSIDES & POSTERS -- At the National Archives through February 1987.