1. Two groups of adult male mice were injected daily with 340 m.u. of an aqueous solution of crystalline oestrin for fifteen days and twenty-one days respectively.
2. Matings with these injected mice were fertile.
3. The testis showed no degenerative changes, spermatogenesis being active in the large majority of seminiferous tubules. The diameter of the tubules showed no decrease in size. The epididymis was distended with spermatozoa.
4. The anti-masculine effects obtained by previous workers have not been confirmed.