The neuroendocrine effects of haloperidol therapy have been examined in 62 male chronic schizophrenic patients, aged 16-62 years. The duration of the disease varied between 2 and 29 years. The patients were divided into 48 hebephrenics with onset of the disease at puberty, or immediately after puberty, and 14 paranoids with onset of the disease in adulthood. They received 6 mg i.m.p.d. of haloperidol, for 30 days, up to a total dose of 180 mg. The following hormonal variables were examined before therapy and at 10-20 and 30 days of treatment: total urinary gonadotropins, serum FSH and LH, GH response to insulin stimulation, ACTH reserve (Metyrapone test), total urinary 17-ketosteroids and 17-hydroxycorticoids before and after an ACTH stimulation test, serum testosterone, insulin response to glucose load, plasma thyroxine before and after a TSH stimulation test. The basic hormonal values revealed decreased secretion of total gonadotropins, FSH, LH, ACTH and testosterone, and increased insulin secretion. The haloperidol therapy seemed to stimulate the secretion of FSH, LH, total gonadotropins, ACTH and testosterone, up to normal or low-normal levels. No modifications were observed in the other hormonal variables. The significance of these results is discussed.