of entrainment of these biological periodicities. Within the range of entrainment the phase relationship between the rhythms of photoperiod and those of testis size and molt changes systematically as a function of the period of the former: as the period of the photoperiodic cycle decreases, the phase of the testicular rhythm (e.g., testicular maximum) and that of the molt rhythm (e.g., onset of molt) become progressively delayed. This behavior is in agreement with predictions derived from general oscillator therory [7] and suggests that circannual rhy thmsl ike circadian r h y t h m s may be described as oscillators in the technical sense.