Male Wistar rats were kept until the end of the second month alive under constant environmental conditions and a rhythmical light‐dark regimen (L/D). Then the animals were divided into 2 groups. While one group of rats continued to live under L/D the others were exposed to continuous illumination (L/L). At the end of the third month alive L/D‐ and L/L‐ animals were decapitated in chance succession at 6 different times of the 24‐hrs period. The pineal glands were removed and extracted. The water soluble extracts were tested for their antidiuretic activity (ADA). With L/D animals the ADA showed a circadian rhythm. The maximum value occurred 3 hrs after the onset of darkness, and the minimum value was found in the middle of the light period. The circadian rhythm was greatly altered when the animals were exposed to a 4 week's period of L/L.