Levels of arginine-vasopressin (AVP) and oxytocin (OXT) in cerebrospinal fluid (CSF) of rats were determined at various times of the day and the night under normal and changed light-dark conditions. During a regular daily 14 h light and 10 h dark cycle (lights on 06.00 h, off 20.00 h), AVP in CSF reached a peak at 13.00 h, while the lowest levels were found at 19.00 h. Reversal of the normal light-dark cycle into a 14 h dark and 10 h light cycle (lights on 20.00 h, off 06.00 h) did not change the normal AVP rhythm. These lighting conditions elevated the OXT levels in the CSF as compared to those found during a normal light-dark regime, but again no differences were observed between the OXT levels determined at the various time-points. A shift of 6 h of the light-dark cycle (lights on 12.00 h, lights off 02.00 h), completely disrupted the AVP rhythm. However, after 3 weeks of adaptation to this new light-dark regime a high level of AVP in CSF was again observed at 13.00 h, and a low level at 19.00 h. The present data suggest that changes in the normal light-dark conditions affect the levels of neurohypophyseal peptides in the CSF. The results are of interest because of reported changes in memory function induced by alterations in the light-dark cycle.