Developer Gerald D. Hines, who has been working on a proposed downtown complex for Washington, plans to begin building a 55-story, 748-foot skyscraper in Houston next month. He says the project, part of a complex called First International Plaza, will the city's tallest.

Houston Oil & Mineral Corp. has leased 600,000 square feet, which Hines says may be the largest amount of lease space ever committed to one firm in the state. The $8 million building, scheduled to open in 1980, was designed by Charles Bassett of Skidmore, Owings and Merrill in San Francisco.

Hines, who also developed Houston's distinctive Pennzoil Place and who is about to undertake a 1.5 million-square-foot complex called One Pillsbury Place in downtown Minneapolis, has an option to develop city-owned property in downtown Washington at 12th and G streets NW as a Metro Center commercial and office complex.

Last week, it was reported that a consulting firm, Morton Hoffman & Co., told the District government it would be unwise to lease space in the Metro Center complex if construction of a convention center begins this year, and even less advisable if the center is postponed. Hines told the District that such a government leasing commitment was necessary to finance the downtown building.