The circadian characteristics of the daily rhythm in oxytocin (OT) concentrations in cerebrospinal fluid (CSF) were studied in the rhesus monkey. Monkeys subjected to constant light or constant dark for periods of 3 to 6 days manifested persistence of the CSF OT rhythm. A 12-hr phase shift in the light-dark cycle resulted in a resynchronization of the rhythm to the new lighting schedule within 3 to 4 days. Altering the daily feeding and care schedule during a period of constant darkness did not alter the expression or timing of the CSF OT rhythm significantly. These results suggest that the OT rhythm is endogenously generated and that the daily light-dark cycle normally synchronizes the rhythm to the 24-hr cycle.