These studies were done to examine the effect of gonadotropin on rat luteal 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity (the rate-limiting step in cholesterol biosynthesis) in ovaries of pregnant mare's serum gonadotropin (PMSG)-human chorionic gonadotropin (hCG) primed rats. Administration of hCG stimulated HMG CoA reductase activity in a time- and dose-dependent manner: significant increases were noted within 4 h, with maximum effects (30-40-fold increases) seen 24 h after hCG (25 IU) administration. This effect was specific in that only LH, of several hormones tested, was as effective as hCG in stimulating HMG CoA reductase activity, and no change in the activity of either liver microsomal HMG CoA reductase or luteal microsomal NADPH-cytochrome c reductase was seen after hCG. The gonadotropin-induced increase in HMG CoA reductase activity seemed to be due to a net increase in enzyme activity, not to a change in the phosphorylated/dephosphorylated state of the enzyme. Pretreatment of animals with aminoglutethimide, an inhibitor of the conversion of cholesterol to steroid (pregnenolone), prevented the hCG-induced rise in HMG CoA reductase activity, whereas treatment with 4-aminopyrazolo[3,4-d]pyrimidine (4-APP), which depletes cellular cholesterol content, led to striking increases in enzyme activity. However, the combined effects of 4-APP and hCG were additive, suggesting that the stimulating effect of hCG on HMG CoA reductase activity is not entirely due to a depletion of cellular sterol content of luteinized ovaries. Similarly, cholesteryl ester and cholesterol syntheses as measured by [14C]acetate conversion were also increased by hCG and 4-APP treatment.(ABSTRACT TRUNCATED AT 250 WORDS)