Urinary chorionic gonadotrophin (UCG) activity was followed in fifteen chimpanzees during pregnancy using a commercial immunoassay for human chorionic gonadotrophin. In one of these chimpanzees UCG levels were determined by a bioassay based on the increase in prostate weight in immature rats. The peak level of UCG excretion as determined by bioassay was 16,700 IU/24 h, and excretion rates remained greater than 1,000 IU/24 h for 14 weeks. The commercial human immunoassay appeared to be an easy and reliable method for detecting pregnancy and predicting parturition in the chimpanzee.