The American dollar suffered its biggest single-day loss in at least 14 years yesterday -- the second day in a row it was clobbered in foreign exchange markets -- as traders exhibited nervousness about the American banking and financial system in the wake of the troubles of 70 thrift institutions in Ohio.

Fear that the closed savings and loan associations there might be symptomatic of more serious problems in the United States, sent gold -- always a refuge in a monetary storm -- soaring $35.70 an ounce on the New York Commodity Exchange to $339 for the biggest daily gain in a decade.

Trading was heavy and hectic, and the dollar price against most major currencies plunged heavily at the end.

According to the Federal Reserve Board, the dollar dropped 2.39 percent against the currencies of 10 other industrial countries yesterday, putting it down 5.54 percent from the all-time high reached on Feb. 25. That was the largest daily decline in Fed records that go back to Jan. 1, 1971. It eclipsed the 2.01 percent drop of Aug. 12, 1981. Nonetheless, the dollar still remains 2.73 percent over its level at the end of 1984, and 81.31 percent higher than it was at the beginning of 1980.

Trade and monetary officials interviewed here and in New York questioned whether the Ohio situation was the real reason or the excuse for the dollar weakness. "We may be at a turning point in the dollar, and the Ohio situation may be the most convenient explanation at hand," one expert said.

C. Fred Bergsten, director of the Institute for International Economics, noted that one element cited for Monday's decline -- the Commerce Department's announcement that the United States has become a net debtor nation for the first time in more than 70 years -- is really old news.

"I don't see any real changes in fundamental conditions," Bergsten said, "but when the market reacts this way, maybe the underlying psychology is changing, maybe it's a straw in the wind."

Describing the surge in gold buying, another trader referred to "the herd instinct. It just fed on itself." As the dollar was setting new records in the past several weeks, gold had plunged to a 5 1/2-year low of $282 an ounce on Feb. 25. But yesterday's price for gold was the best in almost four months.

According to the New York Commodity Exchange, the gain in the gold price was 11.8 percent, more than the previous one-day record of 7.8 percent on Jan. 17, 1980, when gold was rising toward an all-time high of $875 an ounce.

In trying to assess the psychology of yesterday's market, a Frankfurt dealer acknowledged that the S&Ls in Ohio are small institutions that "do not threaten the U.S. banking system. . . .

"But the market is more bearish now, and the Ohio troubles did remind people that the U.S. banking system does have some troubles."

The British pound put on its best show in weeks. Buoyed by a tough British budget statement and by what traders said were firmer oil prices, the pound rose at one time above $1.14, and closed at $1.1360, up from $1.1075 the day before. Later in New York, sterling shot up to $1.1615 from $1.1160 late Monday. Just a few weeks ago, the pound at one point had been as low as $1.0395 and analysts were predicting a drop to parity with the dollar.

Other late dollar rates in Europe, compared with late rates Monday, included: 3.2750 German marks, down from 3.3505; 2.7790 Swiss francs, down from 2.8620; 10.0050 French francs, down from 10.2410; 3.6975 Dutch guilders, down from 3.7920; and 2,074.00 Italian lire, down from 2,120.00.