In male and female rats of two different age groups, we measured the arginine-vasopressin (AVP) levels in cerebrospinal fluid (CSF) collected at different times of the day. Mean values of AVP in cisternal CSF averaged across the different times of the day were higher in males than in females (p = 0.02), and in young (2 months) than in aged (12 months) rats (p = 0.03) of both sexes. The AVP levels in CSF showed a clear circadian rhythm in both young and aged male rats; the values at 07.00 h and 13.00 h were higher than those at 01.00 h and 19.00 h. The amplitude of the rhythm was smaller in females than in males. In addition, the rhythm was more pronounced in young than in aged rats of both sexes. These data suggest that both age and sex affect levels of vasopressin in CSF.