Intact and radiothyroidectomized male starlings were transferred from short (8-hr) to long (16-hr) day lengths and measurements made of testicular development and of plasma prolactin concentrations. In intact birds the testes reached full maturity within 3 weeks and regressed after 6 weeks of photostimulation, indicative of the development of photorefractoriness . Prolactin levels increased gradually, reaching maximal values (25.2 +/- 4.7 ng/ml compared with 1.2 +/- 0.2 ng/ml before photostimulation) at the time of testicular regression. Testicular growth was normal in the thyroidectomized birds, but in three of eight testes remained fully mature for at least 1 year, and prolactin remained low (below 2.0 ng/ml) during this entire time. The testes of the other five thyroidectomized birds did regress partially, accompanied by a corresponding increase in prolactin, although the testes subsequently regrew . Removal of the testes from thyroidectomized birds after 1 year on long days resulted in a marked rise in plasma FSH, indicating that these birds were indeed not photorefractory ; FSH levels in the refractory-intact starlings remained low after castration. These results confirm that the presence of an intact thyroid gland is essential for the development of photorefractoriness and for the release of prolactin which normally follows photostimulation.