• Previous studies showing inconsistent effects of neuroleptic agents on the pituitary-gonadal system suggest that the drugs may differ in their effects on this system. Serum testosterone, luteinizing hormone (LH), prolactin, and neuroleptic levels were measured in 42 male schizophrenic patients during long-term treatment with thioridazine hydrochloride, trifluoperazine hydrochloride, chlorpromazine hydrochloride, and other neuroleptic agents and in six drug-free patients. Serum testosterone and LH values were significantly lower in patients taking thioridazine than in those taking other neuroleptic drugs. The relatively high serum neuroleptic levels in patients taking thioridazine may account for its differential effect on the pituitary-gonadal system.