The National Symphony Orchestra Ball has been running along so smoothly for the last 31 years, it's pretty much a known quantity. For $250 a plate, the 1,200 guests expect to see elegant gowns, chestfuls of sparkling medals, classy decorations, a good meal -- and a good time.

Last night's white-tie gala was no exception. Indeed, the only guest who seemed to deviate from the dress code was former Nixon campaign adviser Clark MacGregor, who came to the Sheraton Washington decked out in a MacGregor tartan kilt.

"He just adores his heritage," said Barbara MacGregor, who left her tartans at home. "But I had to teach him to walk so the pleats would sway open."

And how did the kilted MacGregor respond to the inevitable question?

"What a Scotsman wears under his kilt he wears very proudly," he declared.

"He always says that," responded his wife. "But he really wears jockey shorts."

MacGregor's was not the only heritage represented at the ball, which was sponsored by the French Embassy. Charge' d'affaires Bernard Boyer and Nicole Boyer, along with NSO board president Virginia Mars and ball chairman Margaret Hodges, undertook the monumental task of shaking all those hands in the receiving line, but didn't mind a bit. "It is pleasant to share in the excitement," said Boyer later.

An event like the NSO ball is unknown in Paris, since fund-raising for the arts "is a typical American institution. But every country has its habits," Boyer added diplomatically. The organizers of last night's institution said they expected to top last year's $300,000 mark.

The Boyers' contributions did not end last night, since they will be giving a dinner this evening for the 58 out-of-town guests who each paid $1,500 for a "Symphony Weekend" package that included lunch yesterday in the Executive Office Building hosted by national security adviser Robert C. McFarlane.

"There was a National Security Council person at each table," said one Symphony Weekender, Houston socialite Joanne Herring. "The people who come for this weekend don't come just for the social aspect -- they come to learn something."

Herring's escort, investor Victor Schafferman, came quite a way to learn something. "He flew here on the Concorde from Switzerland," said Herring. "He's really doing it right -- he flew in and had his driver bring his Rolls-Royce down from New York. And the driver was even wearing a cashmere coat. I approve of that."

Other guests who dined on entreco te de boeuf in a ballroom decorated with an Eiffel Tower and poinsettia centerpieces included Attorney General William French Smith and Jean Smith, Walter and Bennetta Washington, Secretary of Agriculture John Block and his wife, Suzanne, and Italian Ambassador Rinaldo and Anne Merete Petrignani, who sponsored the ball two years ago.

"We've been coming for the last 20 years," said Washington. "And when I was mayor, we never missed it. It just keeps getting bigger.

"Now you have to spend 20 minutes in the driveway before you can get in."