The Washington Post Co. yesterday reached tentative agreement to purchase a 20 percent interest in the National Journal, a weekly magazine specializing in politics and government.

The National Journal is published by Government Research Corp., a privately held Washington firm headed by Chairman Anthony C. Stout.

The preliminary plan calls for the Post company--owner of The Washington Post, Newsweek magazine and several television stations--to acquire a one-fifth interest in a new Government Research subsidiary consisting of the National Journal and the Almanac of American Politics, a reference book published every two years.

The purchase price and terms of the acquisition were not disclosed. "The consequence is an infusion of cash" that will allow National Journal to expand, said John Fox Sullivan, president and publisher of the Washington weekly.

"We've got a small company that has been successful and is growing fairly rapidly," he said.

"We are looking to expand even further and for National Journal to grow."

Sullivan said The Post Co. will be only an investor and will not be involved in the editorial direction of National Journal.

With an annual subscription rate of $455 a year, National Journal has a circulation of about 5,000.

A year ago the publication began accepting a limited amount of advertising.

Government Research Corp. also sponsors seminars and does specialized research on public policy issues, but those ventures are not involved in the agreement with The Post Co.