The possibility of the occurrence of rhythmic variations in the activity of the nervous system has been demonstrated by longitudinal studies on male and female subjects. The studies included measurements on ‘absolute’ pitch and simple reaction times. Both sets of measurements showed an approximately two weekly rhythm in the female subject which was linked to the menstrual cycle. In the male subject, ‘absolute’ pitch measurements indicated a rhythm of approximately 20 days in length. The reaction time measurements also demonstrated this rhythm together with a shorter approximately 5 day cycle superimposed. Further studies in which the stimulus for the reaction time was triggered by different phases of the cardiac cycle demonstrated that these two male cycles could be separated from each other.