Quantitative detns. of parenchyma, cortex, medulla, and interstitial tissue of thymus glands from 24 [female] [female] and 29 [male] [male] were carried out by Hammar''s method, but with Heidenhain''s "Susa" as fixative instead of formalin. In [male] [male] thymic atrophy (involution) began at the end of the 2d postnatal month, in [male] [male] in the 4th mo. During the whole 1st yr. the thymus was larger in [female] [female] than in [male] [male] In thymus glands of 4 [male] [male] and 4 [female] [female], about 4 mos. old, the amt. of desoxyribose nucleic acid was detd. (with radioactive P as indicator) and appeared to show a sex difference in lymphocyte production corresponding to the difference in thymic size. Prepubertal castration of 24 [female] [female] and 20 [male] [male] produced a considerable postpubertal enlargement in [male] [male] not in [female] [female]; but in both sexes there was delayed involution.