Summary The impact on wool follicles of development in an athyroid environment was studied in a series of twin fetal lambs by surgically thyroidectomizing one of each pair before the appearance of follicle buds and comparing development of epidermal appendages in it with their development in the normal co-twin. Thyroidectomy was undertaken at 51 to 54 clays' gestation, i.e. after approximately one-third of the gestation period. Each treated fetus was then replaced in the uterus, allowing pregnancy to continue. Eight pairs of twins were removed at intervals from 67 to 122 days' gestation and skin samples from the thyroidectomized and the intact twins were compared. Micromorphometric examination of the samples was used to assess quantitatively the effects of thyroid deprivation on wool follicle development. In thyroidectomized fetuses there was a failure of keratinization in primary wool follicles, an absence of secondary follicles, a tendency to excessive follicular branching and sweat gland development, and a paucity of sebaceous gland formation. The density of wool follicles was substantially increased, but the mean cross-sectional area of these follicles was reduced. The effects of very early thyroidectomy imply that the thyroid plays a role in the stimulation and regulation of wool follicle differentiation. To test the reversibility of the effects observed in the skin of thyroid ectomized fetuses, grafts from these animals were transplanted to normal, young fetal lambs. Subsequent examination of grafted skin revealed that complete keratinization had occurred but that none of the other abnormal features had been reversed.