The fact that it poured and the fact that some of the horses balked at taking the diplomats from their Potomac river cruiser up the path to Mount Vernon didn't deter any of Pepsi Cola's guests last night from having a good, even a historical, time.

As is becoming his custom, Donald Kendall, the chairman of PepsiCo Inc., planned a lavish evening for his friends, Washington politicians and the diplomats from the 144 countries where his soda products are sold. The evening started with a cocktail party for 100 guests aboard the cruiser Diplomat, traveling from Southwest Washington to the historic home of George Washington.

Before the ship left, Sen. S. I. Hayakawa (R-Calif.) was dancing to a Dixieland band with Rita Thapa, the wife of the ambassador from Nepal. At the railing Sen. Larry Pressler (R-S.D.) showed his date, Ava Weir, a real estate agent from New York, the passing sights of Hains Point and Fort McNair.

On the 1 1/2-hour ride some normal Washington business was conducted. Secretary of Health and Human Services Richard Schweiker huddled with Haitian Ambassador Georges Leger, setting up a meeting on the administration's policy toward refugees.

The second phase of the evening, a parade of horse-drawn carriages up the torch-lit road to Mount Vernon, had a few dramatic moments. When New Zealand AmbassadorThomas Gill got about 500 yards in his coach, the horse stopped. An experienced horseman, Ambassador Gill got out and led the horse for a while. "All those lights really confused him. It's a bit dangerous for the horse at night," said the ambassador. The Egyptian delegation got as far as the gate to Mount Vernon and walked the last few yards. "This reminds me of some of our coaches along the Nile. We walked, but it was still exciting," said Aida Chelbaya, the wife of the deputy chief of mission.

Once inside Mount Vernon the diplomats from the boat joined another 150 colleagues and politicians who had taken a land route. Kendall was asking Sen. Strom Thurmond (R-S.C.), "Are you really supporting the sugar subsidy?" Egyptian Ambassador Ashraf Ghorbal was applauding President Anwar Sadat's expulsion of the Soviet diplomats yesterday. "They shouldn't have played that mischief, not in today's world," said Ghorbal. Chinese Ambassador Zemin Chai reacted harshly to the reported evidence of the use of chemical warfare in Cambodia. "It's very cruel," he said.

After another cocktail session, the guests sat down to dinner under a tent large enough to hold 28 tables -- all named for the first 28 American presidents -- the Gene Donati Orchestra and a dance floor. When the rain came down in sheets,the Mount Vernon Guard, a corps of a couple dozen 8-to-14-year-olds, all wearing white wigs and colonial costume, ran for cover and the caterer hurriedly covered the tables set for serving coffee.

No one seemed to mind the rain, which was as plentiful as Pepsi stories. Making a speech for the diplomatic corps, Tanzanian Ambassador Paul Bomani told the story of being home recently. "At a party the people were fussing because it wasn't the right beverage. You can guess what was missing," said Bomani.