[female] albino rats not only show a 4-day estrual rhythm throughout sexual life, but also show long rhythms in activity extending over 200 days or more throughout life. These long fluctuations are also found in [male] rats. On these long fluctuations smaller and shorter spurts of activity are superimposed. The lack of synchronism of these fluctuations in a group of animals indicates that changes in temp., moisture, barometric pressure or season are not the cause, [male] showed a greater tendency toward synchronism than [female]. The fluctuations usually reach a maximum between 200 and 300 days of age, followed by a gradual decline, which is more rapid during senility.