Testicular size and plasma concentrations of FSH and prolactin were measured frequently in thyroidectomized male and thyroidectomized castrated starlings kept in outdoor aviaries during 1 year. In thyroidectomized males, testes increased from minimal to maximal size during March, remained large until June, and then decreased slowly during the next 6 months. Plasma FSH increased to a peak in April (126 micrograms/litre) and then declined slightly. Plasma FSH in thyroidectomized castrates was very high (360-760 micrograms/litre) and did not change significantly during the year. In both groups of birds, plasma prolactin concentration remained low (less than 5 micrograms/litre) and there was no molt. These results demonstrate that while thyroidectomy prevents the onset of photorefractoriness in starlings, short days nevertheless cause gonadal regression. However, this regression is not analogous to the short-day-induced gonadal regression shown by species, such as quail, which do not exhibit absolute photorefractoriness.