Pinealectomy (10 animals) or superior cervical ganglionectomy (14 animals) prevented the acceleration of oestrus in ferrets by two different daily regimes of artificial light. Both sets of animals came into oestrus in the spring following operation at about the same time as normal ferrets kept in daylight. Neither procedure affected the duration of the oestrous period. The interval between onsets of successive postoperative annual oestrous periods in both control-operated and normal animals kept in daylight supplemented by 6 hr. artificial evening illumination was about 1 yr., but was much greater in pinealectomized or ganglionectomized ferrets. An attempt to section the nn. conarii failed to provide direct evidence for a functional link between the superior cervical ganglia and the pineal, but there was a relation between the volume of pineal remaining after this procedure and the activating effect of artificial light on oestrus.