Ralph L. Stanley, viewed by many as an arch-nemesis of the Metro subway system, showed up with hundreds of others yesterday to celebrate Metro's 10th birthday at the Old Post Office Pavilion -- and brought a little present.

It was a savings account containing $25, which, if Stanley has his way as the Reagan administration's mass transit chief, will be the total federal funding for Metrorail in the next fiscal year. Stanley said later that Metro managers could put the money toward repairing the system's rail cars.

On any other day, that might not seem so funny, but Metro officials accepted Stanley's gift in good spirit yesterday and did some poking themselves, warning that the only thing he had any business cutting was the cake.

The birthday party kept about 200 people indoors for about an hour-and-a-half, a feat of major proportions, and provided an excellent occasion for the favorite coinage of governmental bodies, proclamations.

Gov. Gerald L. Baliles of Virginia sent a proclamation, as did Gov. Harry Hughes of Maryland. The Senate of Virginia sent another proclamation that Sen. Edward M. Holland (D-Arlington) noted seemed oddly like the one from Hughes.

Rep. Stan Parris, a Northern Virginia Republican, sent a dollar bill that he signed personally.

It was also an occasion for Metro trivia, such as: What is the maximum speed of the trains (75 mph); what kind of snake was found on a Metrobus (a gabon viper); how long is Metro's longest escalator (213 feet, 10 inches at the Bethesda station); what expensive belonging did someone leave on a train (a $4,000 minicomputer).

With the exception of Stanley, all the various officials pledged to work to see that Metro grows bigger as it grows older until it reaches its full proposed 103 miles.

Mayor Marion Barry, who showed up in cowboy boots with his 5-year-old son Christopher in tow, promised, "We are going to see it finished in our lifetime."

With extension of the Orange Line to Vienna in June, the system will be about 30 miles and an estimated $2 billion short of its goal. Stanley told the crowd that he believed "wholeheartedly that the system will be completed," but he said later that his "emphatic" opinion was "the federal government should not pay for it."