The current investigation studies the effect of a single injection of androgen in the prcpubertal female rat on subsequent development of the endocrine system. A single subcutaneous injection of 1.25 mg. of testosterone propionate was administered to 2, 5, 10 or 20 day old female rats. The animals were allowed to mature to 100 days of age and then were either autopsied or tested for fertility. At autopsy, adrenal and pituitary weights were significantly larger than littermate controls in rats treated at 2 or 5 days of age. No difference in adrenal, pituitary or uterine weight was observed in rats treated at 10 or 20 days of age. Ovarian and uterine weights were markedly smaller in animals injected at 2 or 5 days of age, but not in animals treated at 20 days of age. The 10 day injected group assumed an intermediate position. Corpora lutea were absent from the ovaries of the 2 and 5 day injected groups and from 4 of 10 rats treated at 10 days of age when autopsied at 100 days of age. Breeding studies revealed that administration of androgen at 2 or 5 days of age induced permanent sterility in all animals and in 4 of 10 rats treated at 10 days of age. All animals injected at 20 days of age bore normal litters. It is suggested from these experiments that an androgen-sensitive period exists between birth and the tenth day of age.