Plasma testosterone was estimated by radioimmunoassay in 60 children with disorders of sexual development before and after stimulation with human chorionic gonadotrophin (HCG). In 21 children the testosterone levels after 3 and 5 daily injections of 1000 units HCG were compared and good correlation was found between the paired results (r =0-93), suggesting that the 5-day HCG test has no advantage over the 3-day test. In 7 boys with apparently normal genital development the increments in plasma testosterone ranged from 2-0 to 8-5 nmol/1 after 3 injections of HCG. 10 boys with anorchia showed little response to HCG stimulation, but in patients with other disorders, such as micropenis (10), cryptorchidism (8), hermaphroditism (3), male pseudohermaphroditism (13), hypospadias (3), and sex chromosome anomalies (6), there was considerable variation in the plasma testosterone level after HCG. In 2 boys with suspected anorchia the results suggested that testes were present and this was confirmed at operation.