The prevailing theory of the immature state involves the existence of a hypothalamicpituitary “gonadostat,” which responds with a much greater sensitivity to estrogen feedback, and which results in suppression of gonadotropin secretion in the presence of minimal levels of blood estrogens. This concept has been recently questioned, and soit was decided to re-evaluate the degree of sensitivity in the female rat. Three sets of rats were ovariectomized and given various doses of 17β-estradiol for 5 days postoperative. One set was 26 days of age on the day of surgery, the second 45 days and the third was 95 days old. Animals were bled at the end of treatment, and the sera were analyzed for FSH and LH by radioimmunoassay. Results indicated that the immature (26-day-old) rats were considerably more sensitive to the estradiol than was either group of mature rats, in their suppression of both FSH and LH. Differences in sensitivity between the two adult ages (45 and 95 days) were hardly discernable. We believe that other reported studies demonstrating no difference in sensitivity upon puberty resulted from the failure to provide replacement therapy immediately upon castration. The greater sensitivity of the immature is apparently lost if the animal is allowed to secrete gonadotropins unchecked. Thus, the immature state may require a constant suppression of FSH and LH secretion to be maintained. The effect of aging on sensitivity in the young adult is undoubtedly of minor significance when compared to the great degree of change occurring at puberty. (Endocrinology94:1536, 1974)