Postmaster General Anthony M. Frank, bowing to congressional concern over the printing of U.S. postage stamps in Canada, has promised to "do everything reasonably possible" to end the practice.

His pledge came after the Democratic and Republican leaders of the House Post Office and Civil Service Committee warned they would seek legislation to stop the printing of stamps abroad unless Frank acted. The House members told Frank they were disturbed that the Postal Service had approved printing its "Great American" stamp series in Ottawa.In a letter released by the committee Friday, Frank said the Postal Service "would prefer" U.S.-made stamps and pledged that, with the exception of two stamps now being printed in Canada, "we intend to ensure that all stamps are printed in the United States through fiscal 1992.

"Beyond that, we will do everything reasonably possible to develop and sustain United States stamp sources," Frank said.

Committee Chairman William L. Clay, D-Mo., welcomed Frank's statement as "a good first step," but he added that "we need to work to make sure domestic stamp production is sufficient to meet postal needs."

Postal officials disclosed two week ago that a Fairfax County, Va., firm was printing a 35-cent black-and-white stamp honoring the late Sen. Dennis Chavez, D-N.M., in Canada because it could not find private intaglio printing presses in the United States. The Chavez stamp is the first to be printed outside the country.

Earlier this week, postal officials disclosed that a second stamp, a 75-cent stamp honoring the late Chief Justice Earl Warren, to be issued in June, also is being printed in Canada by Stamp Venturers of Fairfax.

The House committee also criticized the Postal Service for purchasing lapel pins made in Taiwan and toy postal trucks made in China.

In the past, virtually all U.S. stamps were printed in Washington, D.C., by the Bureau of Engraving and Printing, a Treasury Department agency. But last year, after relations between the bureau and the Postal Service became fractious, the two agencies signed an agreement for the Postal Service to turn increasingly to private stamp printers.