Monsanto Co., the diversified chemicals conglomerate, said yesterday that it has agreed to buy G. D. Searle & Co., a pharmaceuticals company that produces and markets the low-calorie sweetener NutraSweet, for $2.7 billion.

Monsanto had long expressed interest in buying a pharmaceuticals company such as Searle to gain the marketing, distribution and product development resources it wanted for rapid entry into the health-care field.

The acquisition also gives Searle access to Monsanto's enormous research capabilities and provides a solution for Searle family members who have wanted to sell their shares in the company.

"Searle will now join forces with a financially strong, large enterprise with common strategic interests and a determination to see that the goals we have vigorously pursued for our company are achieved and exceeded," said Donald Rumsfeld, Searle's chairman and chief executive.

Monsanto earned $439 million on sales of $6.7 billion in 1984, while Searle reported profits of $161.6 million on sales of $1.3 billion.

Monsanto, based in St. Louis, said it will move "as quickly as possible" to make a cash tender offer of $65 per share of all outstanding shares of Searle's common stock.

Certain Searle family members and family trusts have agreed to tender their shares at that price, the two companies said. Together, the stock controlled by the family members and the trusts represents about 20 percent of the company's outstanding common stock. Searle has granted Monsanto an option to buy about 7.7 million shares of Searle's authorized but unissued common stock at $65 per share. That would give Monsanto 32 percent of Searle's stock.

The companies said the offer does not depend on a minimum number of tendered shares. Any shares not purchased through the tender offer will be, will rapidly advance Monsanto's timetable for becoming an important factor in the health-care industry," said Monsanto President and Chief Executive Richard J. Mahoney. "It also will permit us to greatly accelerate the commercializing of new products currently being investigated in research programs within Monsanto and in our joint agreements" with universities.

Although analysts agreed that the merger would strengthen both companies in the long run, they disagreed over whether the price may be too high in the short run.

Earlier this year, Monsanto reportedly bid about $1.3 billion for Searle's pharmaceutical division. Searle management later dropped plans for splitting up the company.

The merger announced yesterday includes "a fair amount of dilution" and "is only a good move if they don't hurt their own shareholders," said Theodore J. Semegran, an analyst with Shearson Lehman Bros.

Melnitchenko said $65 per share is "certainly a fair price."

Monsanto's stock closed yesterday at 50 1/4, up 1/2, and Searle closed at 63 3/4, up 4.