In the present studies, we have investigated the role of human chorionic gonadotropin on the biosynthesis of androgens by placentas and corpora lutea of the pregnant rat. We first sought to compare the effect of hCG on placental and ovarian secretion of androstenedione in vivo. For this purpose either 1.5 IU hCG or vehicle was administered to pregnant rats twice on days 12 and 13 and once on the morning of day 14. Blood was obtained from either the ovarian or the uterine vein. After hCG administration, levels of androstenedione secreted in the ovarian vein increased dramatically, whereas those in the uterine vein declined significantly. To establish that changes in androgen levels in the uterine and ovarian veins are due to changes in biosynthetic activity and also to compare the action of hCG on placentas and corpora lutea, tissues were dissected out from rats treated with 0, 1.5, or 9 IU hCG twice daily and incubated in vitro. hCG administration increased the capacity of luteal cells to synthesize androstenedione de novo by approximately 100% and concomitantly decreased placental secretion of androstenedione by approximately 75%. Addition of high density lipoprotein to the medium enhanced both basal and hCG-stimulated androstenedione production by luteal tissues but had no effect on either basal or hCG-inhibited androstenedione biosynthesis by the placenta. To determine which step in the placental biosynthesis of androstenedione is inhibited by increased levels of LH, we determined the effect of hCG administration, on cholesterol biosynthesis and storage, synthesis of progesterone substrate, and the activities of 17 alpha-hydroxylase/17,20-lyase. hCG did not affect the activities of the rate limiting cholesterogenic enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase, placental content of cholesterol and cholesteryl ester, or the placental production of progesterone. However, hCG did cause a substantial decrease in the activity of 17 alpha-hydroxylase/17,20-lyase enzyme(s); responsible for the conversion of progesterone to androgen. In summary, results of the present investigation demonstrate that increases in LH activity in the circulation act on two different steroidogenic glands to either enhance or reduce androgen biosynthesis. hCG stimulates luteal secretion of androstenedione and simultaneously inhibits placental production of this steroid.(ABSTRACT TRUNCATED AT 400 WORDS)