The in vitro influence of dopamine on the in vitro incorporation of leucine-4,5-3H into rat pituitary gland prolactin and growth hormone was studied. This catecholamine primarily inhibited the release of newly synthesized prolactin from the pituitary gland, but had no effect on growth hormone production. Reserpine and perphenazine, agents that deplete catecholamine stores in the brain, greatly increased the synthesis and release of prolactin but not of growth hormone. Guanethidine, however, produced a slight inhibition in prolactin synthesis. The injection of iproniazid and pargyline, monoamine oxidase inhibitors, had only minimal effects on the in vitro incorporation of radioactive leucine into prolactin and growth hormone. The pituitary glands from rats bearing the prolactin- and growth hormone-secreting tumor MtTW5 synthesized significantly less prolactin than controls. Although no difference in catecholamine concentration was found in the hypothalamus of control and tumor-bearing rats, the injection of the latter group with reserpine or perphenazine restored prolactin to supernormal levels. These data suggest that catecholamines may have a physiological function to regulate prolactin synthesis and release in the rat pituitary gland.