The effect of human chorionic gonadotrophin on uterine adenylate cyclase activity in early pregnancy. The purpose of the present study was to determine the possible effect of hCG on cAMP levels in human endometrium during the first trimester of pregnancy. A non-radioactive cAMP radioimmunoassay was used to measure cAMP levels in uterine tissue obtained from six women (6 weeks gestation) treated with 400 i.u. of hCG daily for 6 days, and from another six women (6 weeks gestation) who had received no treatment (controls). Uterine adenylate cyclase activity was also measured in these same individuals using a membrane homogenate technique. The hCG treatment was found to produce a 2.6-fold increase in cAMP levels, in conjunction with a 1.4-fold increase in adenylate cyclase activity. In addition, both the basal and LH-stimulated adenylate cyclase activities were significantly increased during pregnancy. These results indicate that the stimulation of adenylate cyclase by hCG may be of great importance for the initiation of human pregnancy.