The photoperiodic control of puberty in birds was studied using hybrid red-legged partridges (Alectoris graeca chukar). In the wild partridges first breed at about 10 months of age. Males and females were held from hatch on short (8L:16D) daylengths and then at 3, 4 1/2, 6, 7 1/2, 9, and 12 weeks of age groups were transferred to long daylengths (20L:4D) for 3 weeks to test for the photoperiodic induction of gonadal growth. Control groups were maintained on either the short or long daylengths and killed at 3, 9, and 12 weeks of age. Gonadal growth did not occur in either of the control groups, nor in any of the birds photostimulated prior to 6 weeks of age. However, in birds photostimulated after this point gonadal size was increased in a graded manner, this growth being reflected in the plasma and pituitary levels of luteinizing hormone. Similar long-day responses which increased with age were observed in plasma and pituitary levels of prolactin (PRL). In the control birds there was a steady decline with time in plasma PRL, with no change being seen in pituitary PRL. Hypothalamic gonadotrophin-releasing hormone content did not change in birds maintained on 8L:16D or 20L:4D and was only elevated by photostimulation in the females. Plasma testosterone remained low in all the males except for those transferred to 20L:4D at 12 weeks. Testicular testosterone content remained low in control birds but was elevated by photostimulation at 6 and 12 weeks of age. Plasma progesterone levels increased with age in control birds and were only elevated by photostimulation at 3 and 4 1/2 weeks of age. These data show juvenile partridges to be completely unresponsive to photoperiodic stimulation and in a condition very similar if not identical to that seen in adult refreactoriness. As in the adults, this refractoriness is progressively dissipated by exposure to short days so that by 9-12 weeks of age the partridges are able to respond maximally to long-day stimulation.