Endocrine changes induced by dopaminergic inhibition may play a part in the development of sexual dysfunction (reduced libido/potency) in schizophrenic patients on long-term neuroleptic treatment. Prolactin, testosterone, LH and FSH serum level determinations by radioimmunoassays were performed in 24 schizophrenic males on long-acting fluphenazine decanoate treatment. Of these patients, sexual dysfunction was present in 17 cases, in 10 patients sexual activity remained unchanged. As compared to the 15 healthy controls (7.33 +/- 0.91 ng/ml), a significant increase of prolactin concentrations was found, i.e. 28.2 +/- 5.15 in the impotent groups, and 16.7 +/- 2.12 in the non-impotent group. The difference between the latter two mean values was, however, not significant. In the patients with normal sexual activity the increase of LH concentration was highly significant. It might be assumed that normal sexual behaviour despite neuroleptic treatment and hyperprolactinaemia is related to the high concentration of LH, though the mechanism of increased LH secretion has not been cleared as yet. The testosterone and FSH concentrations showed no differences in various groups.