Du Pont Co., ending more than two decades of efforts to break into the pharmaceuticals business on its own, said yesterday it had agreed to a joint venture with drug giant Merck & Co. to research and market pharmaceutical products made by both companies.

Spokesmen for the two firms portrayed the arrangement as a classic joint venture, in which the two parters would share ownership, management, funding and profits of the new enterprise. But industry analysts said the deal indicated that Wilmington, Del.-based Du Pont, a chemicals and plastics leader, was admitting its lack of success in becoming a major factor in the hotly competitive pharmaceuticals industry.

"Really, Du Pont is giving up. They have tried for two decades to build a pharmaceutical business, either internally or through acquisitions," said Samuel R. Isaly, a drug industry analyst at Mehta & Isaly in New York. "Now they don't have to put any more money into this rat hole."

Analysts said the joint venture, which is scheduled to go into operation early next year pending final negotiations and regulatory approval, represents the latest in a series of consolidation moves in the fragmented pharmaceutical industry.

Over the past couple of years, Rorer Group Inc. has agreed to merge with France's Rhone Poulen S.A., SmithKline Beckman Corp. has merged with Britain's Beecham Group PLC and Bristol-Myers Corp. has acquired Squibb Corp.

Under the terms of the transaction, Du Pont will contribute to the joint venture all of its pharmaceutical products and research, some of its more basic research, and sales, managementand research staff, while Merck will put in a few products that it markets in Europe and invest an undisclosed amount of cash in the venture.

The combined company will have annual sales of about $700 million, a research staff of 1,500, a sales staff of 600 and a research and development budget of $230 million in its first year.

Among the Du Pont prescription drugs to be marketed by the venture will be the analgesic painkillers Percodan and Percocet; and anti-arrhythmic, Ethmozine, and an anti-clotting agent, Coumadin. Merck will provide European marketing rights to Sinemet, a medicine used in the treatment of Parkinson's disease, and Moduretic, a cardiovascular medicine.

"Du Pont is putting {in} the whole thing, and Merck is just putting in some old products," said Hemant Shah, a drug industry analyst at HKS & Co.

The joint venture also will include a group of hypertension compounds developed by Du Pont called angiotensin II receptor antagonists. These drugs, which are now in clinical trials and considered highly promising, already were the subject of an earlier joint venture between the two companies.

The companies will form a joint board of directors to run the operation, which will be called Du Pont Merck Pharmaceuticals Co. and owned jointly by the two companies. Spokesmen for Du Pont and Merck, however, disputed the suggestion that Du Pont was yielding its pharmaceutical business to Merck.

"It's not exiting the business at all. We will continue to put money in it," said Roger Morris, a spokesman for Du Pont. "Basically, what we're doing is accelerating our progress in trying to grow Du Pont Pharmaceuticals into a worldwide pharmaceutical company."