Regulation of human corpus luteum activity was studied with radioimmunoassays and bioassays of sera drawn serially from women suspected of having ectopic pregnancies. Progesterone values exhibited considerable overlap in pregnancies that were subsequently classified as normal intrauterine (n = 21), ectopic (n = 35), or spontaneous abortion (n = 14). The rate of change of human chorionic gonadotropin concentration was significantly correlated with progesterone levels in ectopic (r = 0.64) and all pregnancies (r = 0.70). There was no correlation (r = -0.18) between the rate of change of human chorionic gonadotropin and the bioactivity produced per volume of serum. Ectopic pregnancies and normal intrauterine pregnancies did not differ in their ratio of in vivo bioactivity to immunoactivity. From these data we conclude that corpus luteum activity is primarily regulated by the rate of change of human chorionic gonadotropin concentration, without the involvement of other serum factors. We also conclude that reduced corpus luteum function in ectopic pregnancies is not a result of biochemical modification of the human chorionic gonadotropin molecule. Finally, we discourage the use of single progesterone values in screening for ectopic pregnancies because of the considerable overlap in progesterone values among these and normal intrauterine pregnancies.