In the male black-headed bunting,Emberiza melanocephala, photostimulated testicular and/or body weight growth was followed by the regression. Transfer of photorefractory birds from 20L/4D to 23L/1D or from natural lighting (12–13 h) to 20L/4D failed to evoke testicular and/or body weight recrudescence. Thyroidectomy suppressed light-induced increase in the testes and resulted in early regression. Fully developed testes of breeding birds also regressed following thyroidectomy; an effect which was reversed by daily injections of 1 μg/bird of L-T4. Treatment with L-T4 at doses from 0·5-2·0 μ/bird/day/ 30 days had no effect on the testes of birds maintained on 12L/12D or following a shift from 12L/12D to 20L/4D. Photoinduced increase in body weight was inhibited by thyroidectomy; an effect which was reversed by treatment with L-T4 at dose level 1 μg/bird/day. The extent to which thyroidectomy decreased body weight of birds depended upon the lipid reserves at the time of operation. It is suggested that in the male black-headed bunting (i) breeding is terminated by development of absolute-gonadal and metabolic-photorefractoriness and (ii) thyroid hormones are necessary for sustaining light-induced increase in the gonads and/or body weight and for their maintenance, but not for the development of photorefractory state.