SUMMARY Removal of the pineal gland from ferrets kept entirely in daylight prevented the appearance of the normal annual oestrous rhythm. Animals were operated upon in the autumn; that is, at the start of their anoestrum. Oestrus in pinealectomized ferrets began at the usual time the following spring, as in control-operated and normal animals. However, during the second year after operation, pinealectomized ferrets came into oestrus 20–30 weeks after controls. Mean intervals between onsets of successive oestrous periods were: pinealectomized, 73·8 weeks (4 animals); control-operated, 52·9 weeks (7 animals); and normal animals, 52·5 weeks (10 animals). The duration of the oestrous period was not altered by removing the pineal. Three other pinealectomized ferrets kept in artificial photoperiods (14 h light: 10 h darkness) showed an oestrous rhythm indistinguishable from those pinealectomized and kept in daylight, whereas four controls that had also been put into artificial light showed greatly prolonged oestrous periods lasting more than a year. These experiments indicate that the pineal is concerned with timing the annual breeding season of ferrets in natural daylight as well as in artificial illumination. The anterior pituitary gland of pinealectomized ferrets seems unresponsive to either natural or artificial light under the conditions used in these studies. The duration of oestrus may be self-limited, but can be prolonged if the pineal gland is stimulated by the artificial 'long day'.