The merchandise trade deficit soared to a record $16.5 billion in July, the Commerce Department reported yesterday, increasing the chances that the trade shortfall for all of 1987 will be the highest ever.

Some of the increase was due to special factors, but there was little sign of wholesale improvement in the trade performance, which economists believe poses a serious threat to the economic health of the nation. Continuous trade deficits can eventually lead to higher inflation or slower economic growth. The trade deficit has risen for four straight months.

However, financial markets responded to the figures with unexpected optimism. Stocks, bonds and the dollar all rose significantly after falling slightly earlier in the day.

Analysts were not sure how to explain the positive response to the negative trade figures.

"Weird, definitely weird. I don't know what exactly is going on," said Robert Giordano, codirector of economic research for Goldman, Sachs & Co.

One possibility, he suggested, was that the markets believed the Federal Reserve would not respond to the news by further increases in interest rates.

"On any given day it's very hard to pin down why prices change, but this could just be a big sigh of relief," Giordano said. "No inflation danger could be worth 30 points on the Dow."

His theory was bolstered by the latest inflation statistics. The Producer Price Index was unchanged in August, the Bureau of Labor Statistics said yesterday, the first time in nine months that prices on the wholesale level did not go up.

The Dow Jones industrial average, meanwhile, closed up 32.69 points, while the dollar rose against major currencies and bonds also ended higher than Thursday's levels.

Imports totaled $37.5 billion and exports were $21 billion in July, the Commerce Department said. The previous record for the monthly trade deficit was $16.1 billion, registered in July 1986.

The third-largest shortfall was the $15.7 billion trade deficit in June of this year. In 1986, the annual trade deficit was a record $156.2 billion.

The Reagan administration sought to put a good face on the deficit figures.

Officials said the recent fall in the dollar's value was partly responsible for the increase in the trade deficit, which is expressed in dollars.

"The president is always concerned about the trade deficit, but the trend lines show we are going in the right direction," said White House spokesman Marlin Fitzwater.

Acting Commerce Secretary Bruce Smart said that trade deficits (which are not adjusted for seasonal"Make no mistake about it, the trade deficit continues to deteriorate. There is no turn in trade." Economist Allen Sinai factors) tend to increase in July, and U.S. Trade Representative Clayton K. Yeutter, while calling the deficit "disappointing," said it showed improvement after adjustment for inflation.

But Democratic congressional leaders, who next week begin negotiations to reconcile the House and Senate versions of trade legislation, reacted to the new figures with dismay.

Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.) said they "expose for all the world to see the glaring failure of this country's trade policy and of its energy policy.

"The numbers for the first seven months indicate that a new record trade deficit is all but inevitable this year," Bentsen said. "Our deficits for the remaining five months of 1987 would have to average less than $11.6 billion a month to avoid it, and that's not a realistic expectation."

Senate Majority Leader Robert C. Byrd (D-W.Va.) said the deficit enhanced the need for "effective cooperation" from the administration in fashioning a compromise trade bill. President Reagan has threatened to veto the measure if it resembles the House or Senate versions.

The Democratic Congressional Campaign Committee sent off 175 Mailgrams to congressional candidates and other political figures urging them to whip up grass-roots sentiment on the trade bill.

Economists said it was unlikely the trade situation would improve much in coming months.

In theory, the decline in the dollar's value should lead to fewer imports, which become more expensive to American buyers, and more exports, which become cheaper to foreign buyers. But structural factors have to a great degree kept that turnaround from taking place so far, economists said.

Many of the countries that would be expected to purchase U.S. exports, for instance, are experiencing slow economic growth themselves.

And Allen Sinai, chief economist for Shearson Lehman Bros., said foreign manufacturers are holding down the prices of their exports to mitigate the effect of the dollar's fall, while U.S. companies are raising their own prices to increase profits. Both actions tend to work against reversing the trade deficit.

"Make no mistake about it, the trade deficit continues to deteriorate. There is no turn in trade," Sinai said. "Our trade deficit is an incredible political and economic problem."

There were some bright spots visible in the figures, however.

David Hartman, chief international economist for Data Resourcescq, pointed out that if oil prices were not included, the trade deficit would be $2 billion smaller than it was in July 1986.

And he said that the recently announced decline in Japan's trade surplus with the United States in August indicated the U.S. trade deficit for August, which will be released next month, might be smaller.