The sleep-related increase of plasma testosterone (T) in adult men appears to be related not only to plasma luteinizing hormone (LH) levels but to prolactin (PRL) levels as well, suggesting that PRL may have a stimulatory influence on Leydig cell function. To further investigate the influence of PRL on T secretion, five young adult men were studied on three separate days one week apart. Blood samples were taken every 20 min between 0900 and 1800. At 1000 on each of the three days they received an intramuscular injection of saline, haloperidol 0.25 mg or haloperidol 0.50 mg, in a double-blind design. The blood samples were analyzed for LH, follicle stimulating hormone (FSH), PRL and T. It was hypothesized that there would be a dose-related increase in both PRL and T following drug administration. Mean PRL levels rose promptly and significantly in a dose-related manner in response to the haloperidol, which has strong dopamine blocking effects. By 1600, PRL had returned to control values. In contrast to the PRL response, neither LH nor FSH levels were affected by haloperidol. On the saline control day mean T levels showed the normal decline during daytime hours. After 0.25 mg haloperidol, mean T levels were maintained for several hours, and after 0.50 mg haloperidol, T levels were increased for several hours. These alterations in the normal diurnal pattern of T were statistically significant. They began about 60 min after the corresponding drug-induced increases in PRL levels. This delay between increased PRL and increased T is consistent with the similar delay between the increases of these two hormones that occur at night during sleep. The results of this study lend further support to the hypothesis that PRL is another pituitary hormone that stimulates T secretion in adult men.