Mammary gland development and secretion in male rats were induced by single injections of reserpine. The lactogenic effect of reserpine was most marked when a dose of 1mg/kg was given. The alveoli and ducts of the male mammary glands were greatly distended with milky secretion. Similar changes in the glands were also observed but in a smaller number of animals when a dose of 0.5-0.75mg/kg was given. Nialamide (250mg/kg) counteracted the effect of reserpine (1mg/kg) when given 16-18 hours before the reserpine injection. A similar inhibition was obtained by L-DOPA (250mg/kg) when injected one hour after the reserpine injection. These results provide the evidence that the male pituitary is capable of secreting prolactin in response to reserpine, and also suggest the involvement of monoaminergic mechanisms in the process of prolactin release in the male rat.