Urinary excretory rhythms were studied in male and female rats during a LD cycle (12:12), under constant light (LL) conditions and in blind rats (DD). During DD the period was about 24.5 h. During LL the period was longer or shorter (the latter only in male rats) than 24 h. However, after about 8 days of LL the period again changed, although the rhythms remained free‐running. There were significant shifts in the acrophases during the free‐ running phase: the direction and magnitude depended on the illumination conditions, the duration of LL and the sex of the animal. The difference in internal phase angles between sodium and potassium increased in female rats during the free‐running phase, but decreased in male rats. The rhythms were less precise in female rats than in male rats, but the periods were similar. These results therefore indicate a difference in the organization of the circadian system between female and male rats.