ST. LOUIS (AP) _ Monsanto Co. will buy the Ortho Consumer Products division of Chevron Chemical Co., the companies announced Thursday.

Monsanto's letter of intent lists a purchase price of about $400 million. The companies said they hope to complete the transaction during the first quarter of 1993.

Ortho makes lawn and garden products. Chevron said the sale is consistent with its strategy of focusing on its core energy and oil businesses.

Chevron, based on San Francisco, said it expected to make a ''significant'' gain on the sale, but it didn't specify a figure.

Monsanto, which reported $9 billion in sales in 1991, owns Nutrasweet Co., the pharmaceutical firm G.D. Searle, and markets weedkillers such as Roundup and Rodeo. The St. Louis-based chemical company announced plans in November to cut 3,200 jobs, sell off some assets and slow research to become more competitive in the soft global economy.

Chevron Chemical Co. is a subsidiary of Chevron Corp.