Presidential adviser Stu Eizenstate almost always looks deadly serious, having as he does the burdens of state on his mind. And oh, did he look authoritative last night, reading lot of "whereases" in front of some 2,500 people.

Only problem was, Mickey Mouse stood hand-clasped behind him -- a somber Mickey Mouse, but Mickey Mouses nonetheless. Pretty soon, young children began gurgling and squealing from the audience. Eizenstat took no notice.

"Resolved by the Senate and House of Representatives of the United States of America in Congress assembled," he read grandly to the youngsters, "that the president is authorized and requested to issue a proclamation honoring the memory of Walt Disney on the 25th anniversary of his contribution to the American dream . . ."

And then began an evening of "Zippety Doo-Da," a human being who sang "All my life, I've spent my nights with dreams of you" to Minnie Mouse as well as dozens of congressmen who collected for this latest public relations and promotion effort of Walt Disney Productions.

The occasion was a quarter-century of Disneyland, celebrated by a show of all-American glitz at Constitution Hall. About half of the Congress and diplomatic corps attended, many trailed by well-dressed children in coordinated plaids. It would have looked like any normal Saturday matinee but for the limousines that waited for the kids out front.

It was just a nice get-together, said the Disney folks, on the occasion of their 25th. "We'd like to share it with a few people," was how the marketing director put it.

Or as the Kids of the Kingdom sang in the closing number of the show: "We hope that you'll be able to come and visit us (lots of fanfare and lights and sparkly costumes here) so we can be one big happy family."

Unlike most official Washington after-five events, there was no cocktail reception squeezed anywhere into the festivities."There would have had to have been some liquor involved," explained Jim Garber, the marketing director, "and we wanted to keep this a family affair."

But afterward, after the last black limousine had whisked away the 9-year-old son of the Yugoslav ambassador (Mickey Mouse speaks Serbo-Croatian back home, explained his mother), the grownups gathered for dinner. And drinks.

This was at the Madison Hotel, at a small private party of about 30 or so hosted by Donn Tatum, chairman of the board of Walt Disney Productions. What he heads is an entertainment conglomerate that made $700 million in revenues in fiscal 1979. He talks like a head of such a conglomerate.

"Disneyland is an institution that's world-known," he said proudly and officially. "It lives, of course, on the goodwill of the public. We'd like to have them remember that it's a nice place to come and visit."

Tatum's guests included corporate folk and members of the California delegation, one of which was musing hungrily about what the menu might contain. d

"Broiled duck and pickled mouse," suggested Sen. Alan Cranston (D-Calif.).

As it turned out, it was rock cornish game hen stuffed with rice, and something called peas a la francaise. Afterward, everybody had cake and ice cream. Vanilla.