The possible involvement of cyclic AMP in the regulation of melatonin production was investigated in cultured Syrian hamster pineal glands. Addition of forskolin, an adenylate cyclase activator, to the incubation medium caused marked increases in both cyclic AMP and melatonin levels in glands collected in the second half of the dark period. 8-Bromocyclic AMP, an analogue of cyclic AMP, also increased melatonin production. The 1,9-dideoxy derivative of forskolin, which is unable to activate the cyclase, was ineffective in stimulating either cyclic AMP levels or melatonin production. These results support a primary role for cyclic AMP in the nocturnal increase of melatonin production in the Syrian hamster pineal gland.