On 16th Street, going north above Meridian Hill, traffic slithered along like Mexican rattlers making their way through swamp land. It broke around the Embassy of Mexico where limousines loiter and taxis deposited well-dressed guests to saunter through the monstrous dark wood doors of the Mexican Embassy.

It was Mexican Independence Day. For some of the several hundred guests juggling social schedules, it was also National Day over at the Embassy of Malaysia on Massachusetts Avenue. For those clumped outside at the bus stop, oblivious to what was going on inside, it was just a cool, gray Wednesday evening.

Richard Berendzen, president of American University, was six events deep last night. "The first was Higher Education, the second was Brookings, the third is this," he said at 7 p.m., "the fourth is Malaysia, the fifth is a speech on campus, the sixth is a radio program on the West Coast." Berendzen, whose university is home to more than 200 Mexican students, accomplishes this on orange juice. He jiggled the ice in his empty glass. "People think I'm a good Moslem."

Host to all this was Mexican Ambassador Hugo Margain, who was clearly delighted with the social madness that included an array of guests from cultural organizations, diplomatic circles, Washington society, and the Reagan administration. Commerce Secretary Malcolm Baldrige was among the most prominent in the last category. Margain stood with his wife, Margarita, at the second-floor railing, remembering all names and native tongues, greeting guests in English or Spanish, wherever appropriate.

How are Mexican relations with the United States, Ambassador?

"Excellent!" he said, practically bouncing for emphasis, perhaps thinking the party was clear evidence of such, although it's unlikely he would have estimated relations at anything less. (At least, not at a party.)

Margain embraced former mayor Walter Washington. The wives embraced each other. "Oh, darling," sighed Bennetta Washington as she hugged Margarita, each in double strands of pearls, "how lovely to see you."

Talk was more of the social vintage and less of the international politics vein. Democratic fund-raiser Esther Coopersmith is still on the job these days. "It's harder," she said. "The Republicans always have an easier time raising money. It's just simpler to be a Republican. I always tell my children, 'If you want to go into politics, be a fund-raiser for the Republicans.' " She looked around the crowd. "Are you going on to Malaysia?" she asked.

Austin Kiplinger, editor of the Kiplinger Letter (and immediate past president of the National Symphony Orchestra board -- "the best title to have") talked with friends about Mexican-American-Canadian relations. "We will never agree completely because we all have varying interests," he said. "The important thing is that we talk to each other. We've been a little haughty in the past -- looked down our noses at our neighbors."

Kiplinger nodded toward Alejandro Orfila, secretary general of the Organization of American States, standing about three feet away in another conversation. "I asked Alex -- 'What should we do?' 'Just listen,' he said."