[female] rats were thyroidectomized through 3 successive generations and bred to litter-mate males. The thyroids of these [female][female] and those of their progeny were treated with osmic acid to show the reticular apparatus of Golgi and studied histologically and cytologically. The growth curve of the litters was taken as a measure of lactation. Thyroidectomy in [female] white rats reduced the growth of their offspring 12.13%. Since the growth of young rats is entirely dependent on the maternal milk, it follows that lactation was in some way affected. The effects on lactation were not cumulative through 3 successive generations of thyroidectomy, nor could any constant histological or cytological changes be demonstrated in the thyroids. The gestation periods of the thyroidectomized animals were prolonged approx. 24 hrs.