ST. LOUIS (AP) _ Monsanto Co., spurred by strong across-the-board gains, on Wednesday reported record sales and profits for the second consecutive quarter.

Net income for the second quarter ended June 30 was $241 million, or $3.54 a share, a 10 percent increase over the same period last year, when earnings were $220 million, or $3.04 a shae.

Revenue for the quarter was $2.35 billion, up 4 percent from the $2.26 billion reported in last year's second quarter.

Richard J. Mahoney, Monsanto chairman and chief executive officer, said the chemical company benefited from diversification, combined with favorable market conditions.

''Monsanto has repositioned itself into a new category of company,'' said Mahoney, ''a company with products that are based on chemistry, but far less affected by traditional chemical industry cycles. Our product portfolio is now much less sensitive to economic fluctuations than in the past.''

For the first six months of the year, Monsanto's profit grew 7 percent to $463 million, or $6.78 a share, from $430 million, or $5.90 a share, in the comparable 1988 quarter. Revenue in the first half rose to $4.6 billion, up 5 percent from $4.4 billion in the same year-ago period.

Monsanto's agricultural subsidiary, Monsanto Agricultural Co., posted record profits during the quarter largely because of increased worldwide sales of Roundup and Lasso herbicides.

In its Animal Sciences division, demand for Alimet animal feed supplement continued to expand, especially in markets outside the United States, but competitive pressures held profits in check.

Monsanto's core chemical business also increased with demand particularly strong in Europe, although auto-related markets slowed.

Results at Searle improved dramatically, due in large part to the ongoing strength of Calan SR antihypertensive drug. In addition, Mahoney said early results for the U.S. launch of Cytotec antiulcer drug were encouraging.

Monsanto's NutraSweet Co. posted a strong quarter, although sales were somewhat lower than last year's record-setting second quarter. But the subsidiary set a sales record for the first half of the year.