Activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase, involved in the biosynthesis of melatonin from serotonin, were assessed over 24 h in the pineal organ of pikes ( Esox lucius, L.; teleosts) entrained to natural (winter) environmental conditions. Only NAT activity exhibited daily changes, rising at the onset of darkness and resuming low values shortly before the end of the scotophase. The rhythm was damped under constant darkness, lower and higher values being shifted towards intermediate ones. NAT activity was lowered under constant light; however, a significant increase was seen at the end of the subjective night, suggesting the existence of a low-amplitude rhythm. Illumination of the pikes at the middle of the scotophase induced inactivation of roughly half of the maximal NAT activity, possibly indicating the existence of one photolabile and one photostable enzymatic component. Under natural conditions, radioassayable melatonin of the pike pineal organ displayed daily variations which paralleled those of NAT activity. Melatonin production thus appears to reflect the daily changes of NAT, synchronized to the light/dark cycle.