The dollar fell through the 200-yen barrier today to hit its lowest mark in nearly two years and predictions that it wil drift even lower in the coming months.

Only a large-scale dollar-buying intervention by the Bank of Japan prevented a steeper plunge as banks and many foreign exchange traders moved out of dollars and into yen for the second straight day.

Market experts cited the slight lowering of the prime interest rate in the United States as the immediate reason for the speculative decline. They also said that the basic strength of the Japanese economy and optimistic predictions for business for 1981 abetted the movement.

The dollar closed on the Tokyo Exchange at 199.60 yen, down 1.80 yen for the day, and would have sunk even lower had Japan's central bank not intervened. It was the lowest point for the dollar since February 1979. It has fallen 60 yen since a high point of 260 last April, a decline of more than 20 percent.

Meanwhile the pound made its best showing against the dollar in two months in response to lower U.S. interest rates, United Press International said yesterday.

Gold closed in Zurich at $599.50 an ounce, a $2 gain on Monday, and at $601.50 in London, also a $2 gain on the day.

The close in New York was $598, up from $594 at Monday's close, and the Commodity Exchange settlement price was $597.80, up from $593.60.

Silver rose to $16.38 an ounce from $16.30 and settled on the Comex at $16.29, up from $16.26.

Dealers said gold's firmness reflected lower money market rates even though the shortest-term bank rates -- the federal funds -- were trading around the 20 percent level.

In London the pound rose to its best level since Nov. 6, closing at $2.4255, up from $2.4165.

The dollar continued to weaken in Europe because of falling U.S. interest rates, but it came back a bit in U.S. trading, partly because of a firming in the federal funds rate.

UPI quoted the following European closing prices, with late New York prices in parentheses:

Frankfurt, 1.936 marks, down from 1.9485 Monday (1.9375); Zurich, 1.74855 Swiss francs, down from 1.7567 (1.7525); Paris, 4.4725 francs, down from 4.485 (4.479); Brussels, 31.305 Belgian francs, down from 31.555 (31.17); and Milan, 919.05 lire, down from 928.80 (921.50)

The 200-yen mark once was regarded as a crucial psychological barrier in the Japanese market, a danger point that the government and businessmen were reluctant to yield. Trading companies and manufacturers fear that if the yen appreciates too much, Japanese products will lose their price competitiveness overseas, causing exports to fall.

The 200-yen level has lost some of its significance in recent years, but the fear of breaking through it was strong enough to cause the central bank to spend heavily to buy up surplus dollars. One trading expert estimated the bank bought between $500 million and $600 million yesterday and may have bought nearly as much again today.

Some economic experts said today that the yen's steep appreciation will have a deflationary effect and called on the government to take immediate steps to lessen it. Toshio Komoto, director general of the economic planning agency, called for an easing of credit, suggesting that the central bank cut the official discount rate. He said the yen's appreciation will have a serious impact on the business of trading companies, particularly smaller ones.

David Bussmann, economic analyst for Bank of America's Tokyo office, said the speculative plunge this week was touched off by the slight decline in prime rates charged by leading American commercial banks.