A quantitative assessment has been made of the height of the cells of the follicular epithelium of the thyroid gland of the Minnow, Phoxinus phoxinus L., as an index of the activity of the gland at different seasons of the year. It is possible, despite the considerable degree of variation which occurs, to demonstrate siginificant fluctuations which indicate that the activity is at its maximum during the early months, from about February to May, during which period the maturation of the germ cells is known to be completed. The maintenance of minnows at constant temperatures for fifty-seven days gives no evidence of an inverse relationship between temperature and cell-height such as is known to exist in certain homoiotherms, but actually suggests a direct one. It seems unlikely, from the available data, that, the thyroid fluctuations can be a response to temperature changes in the environment, and it is concluded that there is an indication in the minnow, as elsewhere in the vertebrates, of some correlation of the activity of the thyroid gland with the phases of the reproductive cycle, the causal basis of which is, however, obscure.