The role played by postsynaptic alpha-adrenergic receptors in the stimulation of pineal melatonin production was investigated in the Syrian hamster. The studies were conducted using organ cultured pineal glands collected from both anatomically intact and superior cervical ganglionectomized hamsters. Results obtained indicate that phenylephrine, an alpha-adrenergic agonist, by itself has no effect in promoting melatonin production; however, it potentiates the stimulatory effects of isoproterenol, a beta-adrenergic agonist, on pineal melatonin production in nonoperated hamsters. Similar observations were obtained with pineal glands whose presynaptic terminals were removed by prior superior cervical ganglionectomy. However, a longer incubation time was required (4-6 hours vs. 2 hours) with pineal glands taken from ganglionectomized animals. Apparently, beta-adrenergic activation is an absolute requirement to stimulate pineal melatonin production, and an alpha-adrenergic receptor mechanism potentiates beta-adrenergic activation. In addition, the findings obtained with denervated pineal glands suggest that the regulation of pineal melatonin production by both alpha- and beta-adrenergic mechanisms is through receptors located on postsynaptic structures.