1. Seasonal variations in the activity of the thyroid gland in yearling brown trout were measured by two methods; first, by the changes in thyroid epithelial cell height and, secondly, by the rate of loss of radio-iodine from the gland.
2. Peak thyroid activity was found to occur in spring with a second peak demonstrable by the radio-iodine technique in mid-summer.
3. With the exception of this short burst of activity in July, the activity of the thyroid was found to be inversely proportional to the water temperature.
4. It is suggested that the thyroid is concerned in a temperature-compensating mechanism.
5. It is suggested that the rate of change of the thyrotropic hormone level in the blood determines the mode of response of the gland.