Three male patients with pituitary tumours and marked hyperprolactinaemia were investigated. Their prolactin (PRL) levels ranged from 210 to 2500 ng/ml. The subjects had clinical and laboratory characteristics of hypogonadotrophic hypogonadism. All were treated with human chorionic gonadotrophin (HCG) and in one subject human menopausal gonadotrophin (HMG) was given in addition. In all three patients, despite the persistence of hyperprolactinaemia, serum testosterone had risen to normal levels within 4--17 days after starting HCG. Despite the normal testosterone level, impotence persisted in two patients and the third had persistently decreased libido. The hypogonadism in these patients may be related to an absolute reduction in gonadotroph number secondary to destruction by tumour mass. Alternatively, hyperprolactinaemia may inhibit the synthesis or release of the gonadotrophins or LHRH. Despite hyperprolactinaemia, pharmacological doses of HCG induced testosterone secretion in all these three subjects.