Differences in the thymus wt. were observed, only when castrated or adrenalectomized animals were compared with "control animals" which were below their proper body size. The thymus wt. in relation to body wt. of optimal growing rats. or of such animals which recovered fast enough from previous growth restraint, was not different from that of castrated or adrenalectomized animals. In the o albino rat the normal course of thymic development is not changed after castration or adrenalectomy. In castrated or adrenalectomized animals under suboptimal conditions, the thymus shows less under-development or atrophy than does the thymus of non-operated animals under similar conditions.