The thymus of castrated [male] and [female] albino rats was heavier than that of controls, with the wt. increase resulting from actual parenchymal hypertrophy. The shape of the thymus wt. curve was not influenced by castration, and no hlstological differences were found. Inj. of testosterone propionate, estrone, or estradiol benzoate caused atrophy of thymic parenchyma in normal and castrated rats of either sex. Thyroidectomy decreased the size of the thymus already hypertrophied as a result of prepuberal castration.