The activity of serotonin N-acetyltransferase, the key enzyme of melatonin synthesis, shows a marked circadian rhythm in the pineal glands of various animal species. The regulation mechanism of the N-acetyltransferse rhythm in birds is different from that in mammals. N-Acetyltransferase activity in rat pineal gland is controlled by the central nervous system through the sympathetic nerves from the superior cervical ganglion, while in chicken the endogenous oscillator for N-acetyltransferase rhythm is presumably located in the pineal gland. Recently it has been shown that N-acetyltransferase activity oscillates in a circadian manner in the organ culture of chicken pineal glands. When chicken pineal glands were organ-cultured under continuous illumination, the nocturnal increase of enzyme activity was suppressed. These observations suggested that chicken pineal gland contains a circadian oscillator, a photoreceptor and melatonin-synthesising machinery. A central question arises whether the circadian oscillation of N-acetyltransferase activity and its response to environmental lighting are generated within the cell or are emergent properties of interaction between different types of pineal cells. I report here that in the dispersed cell culture of chicken pineal gland, N-acetyltransferase activity exhibits a circadian rhythm and responds to environmental lighting in the same manner as in the organ culture.