Testes from hypogonadal (hpg) mice transplanted under the tunica albuginea of the testes of normal mice displayed full spermatogenic activity after 84 days. When ovaries of hpg mice were transplanted to the periovarian capsule of ovariectomized normal females ripe follicles and corpora lutea developed. Although small remnants of normal ovary were found after this operation, the fact that 5 out of 11 normal females bearing transplanted ovaries and mating with heterozygous males raised litters containing hpg mice shows that the ovary of the mutant is capable of producing ova which can be fertilized.