A sharp plunge in the dollar's value yesterday, accompanied by a steep drop in stock and bond prices, prompted a new statement of concern from the White House but no sign of activity by the Reagan administration to stem the dollar's slide.

With the Dow Jones industrial average down nearly 70 points in early afternoon trading and the dollar's value at new lows compared with the Japanese yen and West German mark, the Reagan administration released a statement saying that "the United States wants to see stability in the dollar."

In a statement issued in Los Angeles, White House spokesman Marlin Fitzwater said, "We feel strongly that any further decline or excessive fluctuation could be counterproductive."

But those words merely duplicated a key portion of a communique issued last Tuesday by the United States and its six major economic partners suggesting that the dollar had declined far enough -- a statement that financial markets had not found convincing. The dollar has continued its decline since that communique was made public.

Fitzwater's comment appeared to slow the dollar slide on U.S. markets yesterday and sparked a modest stock market rebound. But traders here and abroad predicted that -- failing some concrete action to make the dollar more attractive to currency traders, such as an increase in U.S. interest rates -- the slide would resume today.

In Tokyo, a reported modest intervention by Japan's central bank helped stabilize the dollar at the close of trading this morning, The Associated Press reported.

On Wall Street yesterday, the Dow Jones industrials closed down 56.70 points, at 1942.97, the 14th worst drop in history in terms of number of points. The 2.84 percent decline was the 12th biggest.

The White House response was issued from the West Coast, where President Reagan was beginning a week-long vacation. A White House official, who spoke on condition that he not be identified, said there was "serious concern" about the dollar's continuing decline, which is boosting prices of imported apparel, equipment and other foreign goods, adding to inflationary pressures in the U.S. economy.

Another senior official said yesterday that the administration is "disappointed" in the slide of the dollar in the wake of last week's communique by the Group of Seven industrial nations, which was meant as a show of support for the dollar.

"We do care about the dollar exchange rate, we are not indifferent to it. The concern of foreign {officials} about the dollar slide is correct," he said.

But the official added that policymakers have no good idea of how to counter the market's perception that the administration is willing for the dollar to decline, regardless of what the statement says.

The stock market slide ended a mild euphoria on Wall Street, which in the past three weeks had enjoyed the best sustained rally since the October crash. But the bad news on the dollar, bringing with it a new rise in interest rates, caused new worries about what lies ahead for 1988. Long-term Treasury bonds dropped about $10 for each $1,000 in face value, which pushed interest yields over the 9 percent level.

Yesterday, the dollar broke to new lows, dropping to 123.20 yen in Tokyo before closing at 123.55 yen, off sharply from 125.20 yen on Friday. In Europe, the dollar tumbled to 1.5940 German marks -- breaking under a 1.60 psychological barrier, down from 1.6275 marks on Friday.

At the time of the Group of Seven statement, which appeared to be pinpointing a new floor for the dollar, the dollar was worth about 126 yen and 1.63 German marks. But sources told The Washington Post that these were not meant to be rigid levels, noting that the language of the statement -- repeated by Fitzwater yesterday -- said that a further decline "could" be counterproductive, not "would" be counterproductive.

Some market analysts believe that the United States may not engage in a vigorous defense of the dollar, through heavy purchases of the U.S. currency, until the dollar dips to about 120 yen and 1.55 German marks. Privately, government sources deny that this is correct, insisting that the administration genuinely wishes -- at this point -- that the decline would come to an end.

Yesterday's dollar trading was very thin, making it difficult to analyze "exactly why it is happening," a trader in Frankfurt said. "But it is evident that the coordinated central bank intervention was insufficient to do more than temporarily prevent the dollar from falling to lower levels."

At yesterday's low levels, the dollar had lost about half of its 1985 value, compared with Japanese, German and other European currencies.

Yesterday's dollar slide extended events that began over the Christmas weekend, when foreign exchange markets began to display a decisively negative reaction to the G-7 statement. Traders made clear that they concluded that there was little in the way of new policy actions to stem a decline.

Fitzwater added no new assurances yesterday, sticking exactly to the language of the G-7 statement. Asked to explain how the administration explains the decline of the dollar and of the stock market, Fitzwater said: "We don't do psychoanalysis."

Since autumn of 1985, the administration has been seeking a decline in the dollar in the belief that a cheaper exchange rate would help American manufacturers sell more goods abroad, while making imported goods more costly, thus reducing a worrisome, damaging trade deficit.

Treasury Secretary James A. Baker III has recently agreed with critics who argue that a further decline could be "counterproductive," boosting values of the Japanese yen and German mark to the point that those economies suffer -- losing more of their ability to buy American goods.

President Reagan has said twice since mid-November that the dollar had fallen enough -- temporarily reassuring markets. But when the president's verbal assurances were not backed up with action, the dollar continued to slide.