Abstract We have recently described a system, using dispersed chick pineal cells in static culture, which displays a persistent, photosensitive, circadian rhythm of melatonin release, and the effects of light and darkness upon it. Here, we describe the effects of forskolin (FSK), a specific activator of adenylate cyclase, 8-bromocyclic AMP (8BrcA), an analogue of cyclic AMP, and 8-bromocyclic GMP (8BrcG), an analogue of cyclic GMP, on melatonin output by these cells. FSK stimulated melatonin output; it was potent and effective. 8BrcA, but not 8BrcG, also markedly stimulated melatonin output. These results support a role for cAMP (but not cGMP) in the regulation of melatonin production. Four hour pulses of white light or darkness, in otherwise constant red light, cause, in addition to acute effects, phase-dependent phase shifts of the melatonin rhythm in subsequent cycles. Such phase shifts indicate an effect on (proximal to) the pacemaker generating the rhythm. Four or 8 hour pulses of FSK, 8BrcA, or 8BrcG, however, did not appreciably alter the phase of subsequent melatonin cycles. Neither did they interfere with phase shifts induced by light pulses. These results fail to support a prominent role for cAMP or cGMP levels in regulating the pacemaker; nor do these cyclic nucleotides appear to mediate the prominent effects of light and dark on the pacemaker. Thus, cyclic AMP regulation of melatonin appears to occur distal to the pacemaker.