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Gonadectomized male Japanese quail maintained on short daylengths (8 h light: 16 h darkness) showed a reliable and repeatable response, in terms of increased gonadotrophin secretion, to a single long photoperiod (20 h light) followed by continuous darkness. Plasma levels of LH were significantly increased within 4 h of the end of the long day. Secretion continued to increase with time and LH and FSH concentrations reached a maximum some 24 h later, thereafter decreasing only slowly over the next 8–10 days. Quail could be used repeatedly at fortnightly intervals and gave a consistent response for at least 15 months. The size of the photoperiodic response and its considerable duration were much greater than found in intact quail. The birds showed a similar (though reduced) response if a short daylength was followed by a single 4-h light pulse interrupting the 16-h dark period. Using this system the time of maximum responsivity to the inductive effects of light lies 10–16 h from 'dawn'. The marked and rapid effects described provide an excellent experimental system in which to study how photoperiodic changes are recognized and how consequent neural activation is translated into enhanced secretion rates of hormones in the peripheral endocrine system.