PITTSBURGH (AP) _ H.J. Heinz Co. stock fell sharply on Wall Street after the food giant announced first-quarter earnings that were well below analysts' expectations.

The ketchup, tuna and cat food maker Tuesday said that profits rose 78 percent as a $221 million pretax gain offset a 3.8 percent decline in sales and a stronger U.S. dollar, which makes overseas sales worth less.

Heinz Chairman Anthony J.F. O'Reilly said the company would use the benefits of a previously announced restructuring to go ''on the attack'' and market core products more aggressively. He said some jobs would be lost in the cost-cutting but wouldn't say how many or when.

The company said profits grew to $254 million, or 95 cents a share, in the three months ended July 31, compared to 143 million, or 54 cents a share, in the same period of 1990.

The gain came from the June sale of The Hubinger Co., a corn milling operation in Iowa.

Operating income grew 63 percent, to $434.4 million.

Sales fell to $1.5 billion from $1.55 billion, reflecting the divestitures of Hubinger and Caribbean Restaurants Inc., which was sold in April 1991.

The earnings report hurt Heinz stock, which fell $2.50 to $38.25 Tuesday on the New York Stock Exchange.

''Things weren't terrible, but this is not an H.J. Heinz report,'' said Greg Drahuschak, an analyst with Butcher & Singer in Pittsburgh. The company has reported growth in sales, net income and earnings per share for the past 27 years.

He said the figures caught Wall Street by surprise, perhaps because the effect of weaker foreign currencies was underestimated.

Drahuschak also said growth potential may be limited for some of Heinz's more mature businesses, and the price of the stock may decrease slightly to reflect that.

''There's only so much ketchup you can eat, and there's only so many players in the ketchup game,'' he said. ''Traditional Heinz products will hold their own, but how much growth can you expect to see out of that?''

Heinz also makes and markets other condiments, StarKist tuna, 9 Lives cat food, Ore-Ida potatoes and Weight Watchers diet products.

Heinz recently bought JL Foods, a John Labatt Ltd. subsidiary based in Eugene, Ore., but the purchase was closed on Aug. 23 and its sales won't be included until the second quarter.

O'Reilly, at Heinz's annual meeting, said the gain from the sale of Hubinger would give the company ''a unique opportunity to go on the attack.''

He said the company will use the money to increase marketing for its major brands, including StarKist, 9 Lives and Weight Watchers, ''where competitive pressures have been intense.''

O'Reilly said the company would accelerate its cost-cutting initiatives in ''relentless pursuit of low-cost operator status.''

The moves would help the company deliver full-year results in line with past trends, he said. He said the acquisition of JL Foods would enable Heinz to continue growing in the 1990s.

At a news conference, O'Reilly said some Heinz workers probably will be laid off. He declined to say how many workers would lose their jobs or when, but said the company's largest operations likely would be affected.

Those operations include Heinz USA and the Ore-Ida frozen potatoes, Weight Watchers brand and StarKist tuna divisions in the United States as well as operations in Italy and the United Kingdom.

Heinz employs about 40,000 people worldwide.

Sales volume increased for StarKist but fell for Ore-Ida frozen potatoes, Weight Watchers brands and 9-Lives canned cat food.

Last month, Heinz stock rose in heavy trading amid rumors the company could be the target of a takeover attempt. The company later said the rumors were false.

Meanwhile Tuesday, Heinz raised the quarterly dividend paid on its common stock to 27 cents from 24 cents per share. The dividend will be paid Oct. 10 to shareholders of record on Sept. 23.