Pineal glands and dissociated pineal cells exhibit a circadian rhythm of melatonin secretion in vitro which persists for several cycles under constant conditions. It is not known whether individual, physically isolated pineal cells are capable of generating a circadian oscillation in melatonin release. This question was addressed by utilizing a reverse hemolytic plaque assay for the detection of melatonin secretion from individual pineal cells. Dissociated pineal cells from the anole lizard, maintained in short term culture, displayed a marked variation in melatonin secretion for up to 72 h under both cyclic lighting conditions and in constant dark. The persistence of daily fluctuations of melatonin secretion from individual cells strongly suggests that individual pineal cells can function as circadian oscillators.