In order to investigate if castration cells or hypertrophic gonadotrophs are induced by ovariectomy at birth, neonatal female rats were ovariectomized within 12 h after birth and the pituitary cells were examined when the rats were 15 days of age by means of immunocytochemistry and electron microscopy. Hypertrophic gonadotrophs were present in the anterior pituitary glands of these young rats. However, when estradiol was injected on alternate days, beginning on the 5th day after birth, this castration response in the pituitary was markedly suppressed. This morphological evidence confirms physiological results previously reported that a feedback interaction between the ovary and the hypothalamo-adenohypophysial system is functional in the early days of life.