The hypothesis that thyroid hormones are required for the expression of normal cycles of reproductive activity was tested in Soay rams, a widely used subject for studies on seasonality. Six rams were thyroidectomized and six sham-operated in October, just before the expected peak of reproductive activity. All were then placed in long days (18 h light and 6 h dark, 18L:6D) for 12 weeks and then transferred to short days (8L:16D). All animals exhibited an initial short-term increase in testicular size and serum concentrations of follicle-stimulating hormone and luteinizing hormone then, as expected, these parameters decreased in sham-operated animals for the remainder of the time they were maintained on long days. Four weeks after transfer back to short days, gonadotrophin concentrations in the control animals were significantly raised above the lowest level, and scrotal size increased after eight weeks. By contrast, in thyroidectomized rams all parameters remained high throughout the experiment. Thyroidectomy therefore abolished seasonal changes in reproductive activity, indicating a requirement for thyroid hormones for the normal expression of such patterns. It is unlikely that this was related to any generalized metabolic disturbances caused by thyroidectomy, as body masses were similar in both groups of animals throughout the experiment.