Supplements of vit. A administered to rats receiving a diet devoid of vit. A prevented the appearance of xerophthalmia in both control and thyroidectomized animals. However, supplements of carotene prevented the ocular changes only in the control rats and not in thyroidectomized rats. The mortality and changes in wt. of the thyroidectomized animals receiving carotene also indicated a decreased utilization of carotene. It is apparent that the thyroid gland plays a major role in the conversion of carotene to vit. A.