Serum levels of FSH, LH, chorionic gonadotropin (CG), prolactin, estrone (E1), estradiol-17Beta (E2), estriol (E3) and progesterone were measured at 2-3-day intervals in 4 chimpanzees through 2-3 menstrual cycles, and serially through subsequent pregnancies. The hormone patterns of the menstrual cycles were similar to those in man, with high levels of FSH in the early follicular phase, followed by rising E2 concentrations to a peak (up to 35 ng/dl) at or just before a midcycle LH/FSH peak. In most cycles there was a secondary E2 rise and progesterone rose to values above 500 ng/dl during the luteal phase. There was no consistent pattern in prolactine levels through 3 menstrual cycles. A simultaneous increase in E2 and LH/CG levels and a fall in FSH about 10 days postovulation indicated fertilization and implantation. Other early signs of pregnancy were persistent luteal range progesterone concentrations and rising levels of E1 and E3. Peak CG levels (56-154 IU/ml) occurred 30-50 days after the midcycle LH/FSH peak, followed by a decline and then a small secondary rise to (to 1 IU/ml) before term. E1, E2 and E3 levels rose more rapidly after 80 days to a peak at term (E1: 180-300 ng/dl; E2: 500-800 ng/dl; and E3:400-1000 ng/dl). Progesterone levels showed one peak coincident with the CG peak, and a secondary rise after about 80 days to maximal values at term of 49-120 ng/ml. Prolactin levels increased during pregnancy with irregular fluctuations (7-127 ng/ml). These findings indicate in contrast to observations in rhesus monkeys and baboons, that the hormonal patterns during pregnancy in the chimpanzee are remarkably similar to those in man. Thus, the chimpanzee should prove to be an ideal model for research directly applicable to human reproduction.