Correlation between secretion of testicular steroids and plasma prolactin (PRL) levels, before and during bromocriptin treatment, was studied in 20 psychiatric patients under neuroleptic therapy for two years or longer. Eleven of them were under additional treatment with antiparkinson drugs (AP group). Plasma PRL, testosterone (T), 5 alpha dihydrotestostérone (DHT), 17 beta-estradiol (E2), 17 alpha OH-progestérone (17 alpha OHP), and dehydroepiandrosterone-sulfate (D-S) were measured by specific RIA both at basal level and in response to testicular stimulation by hCG. Mean basal PRL levels were normal in the patients under neuroleptic treatment along (Ne group), and slightly elevated in the AP group. In the Ne group, an unexpected, significant increase occurred in mean plasma PRL during the hCG stimulation, before bromocriptine treatment. Mean basal steroid levels were normal in both groups. The testicular responses to hCG, as reflected by the T, E2, 17 alpha OHP, and DHT mean plasma levels, were within the normal ranges in the AP group; in the Ne group, however, T and DHT displayed a subnormal mean increase, while E2 and 17 alpha OHP responses were within the normal range. These results suggest that some modifications of the enzymatic activity for testicular steroidogenesis could be induced in the patients under neuroleptic treatment alone. Moreover, a significant reverse correlation was found between PRL and T basal in both group; this correlation disappeared during the bromocriptine treatment.