An exchange of television station WTOP-TV here for WWJ-TV in Detroit was approved yesterday by the Federal Communications Commission.

The regulatory agency's decision means that The Washington Post Co., owner of WTOP, soon will be without broadcasting operations in the metropolitan area for the first time since 1949.

A spokesman for The Post Co's broadcast division said yesterday that the Washington-Detroit station swap probably will take place about July 1. At that time, the Evening News Association of Detroit will own WTOP-TV and The Post Co. will take over at WWJ-TV.

The new owner of Channel 9 here owns Detroit's afternoon newspaper, the Detroit News. As part of the exchange agreement, the Washington communications company also will pay $2 million in cash to the Evening News Association.

Yesterday's unanimous FCC approval followed a decision by the agency last week, supporting the sale of WTOP radio to the Outlet Co., of Providence, R.I., for $6.675 million in cash and notes. That sale also is expected to be closed about July 1.

In 1971, The Post Co. donated WTOP-FM (now WHUR-FM) to Howard University.

The FCC noted yesterday that the Post Co.-Evening News transaction represented "voluntary compliance" with commission policy designed to encourage separate ownership of newspapers and broadcast stations in the same markets.

Post Co. Chairman Katharine Graham said at the firm's annual meeting recently that the decision to sell WTOP-TV and WTOP radio was made "with deep regret."

The sales were necessary because of uncertainties about future government policy on media ownership, she stated. In the near future, the Supreme Court is expected to rule on a case involving the FCC's current policy, which is to prevent future joint ownerships in the same market but to allow most current newspaper-broadcast combinations to continue.

By adding a Detroit station, the Post Co. is moving into a larger market. The Michigan city is the nation's seventh largest for television viewers and Washington is eighth.

WTOP-TV Vice President and General Manager James Lynagh and news Vice President James Snyder will move to Detroit when the sale is completed, to assume management posts at WWJ-TV. Post-Newsweek Stations, the Post Co. broadcast division, is seeking new corporate offices here to replace current facilities at Broadcast House, 4001 Brandywine St. NW, which will become the property of the Detroit firm.