Mean (± SE) body mass, testis volume and feather papillae regeneration in response to treatment with melatonin and prolactin (group 1), prolactin alone (group 2), or vehicle (group 3) in the redheaded bunting (Emberiza bruniceps) subjected to natural day lengths at 27°N (experiment 1: a-c) for 23 days (from mid-June to early July) or to artificial day length (14L:10D; experiment 2: d-f) for 22 days. Birds were injected with exogenous hormones at a dose of 20 μg bird-1 day-1 in 200 μl of vehicle daily for first 10 days in the experiment 1 and for first 13 days in the experiment 2, and thereafter they were left uninjected. Controls received similar volume of vehicle. All injections except melatonin were made at the zeitgeber time 10 (ZT 0 = the time of sunrise for the experiment 1; the time of onset of light for the experiment 2); melatonin was injected at ZT 9.5. Day 0 on X-axis refers to the day before first injection. Under NDL, one bird of group 1 and two birds of group 2 died, and their data are excluded. Asterisk indicates the significance of difference at P < 0.05.