To study the maturation of inhibitory influences on growth hormone (GH) secretion the effect of ether stress on plasma GH levels was studied during postnatal ontogenesis in female rats. Ether stress did not affect plasma GH levels in 1-day-old pups. A distinct decrease of plasma GH was found in 3- and 9-day-old pups, and the response was prevented by treatment of 3-day-old animals with somatostatin antiserum. No effect of ether stress on plasma GH was noted in 12-, 15-, 18- and 21-day-old rats. Treatment of intact 12-day-old pups with the somatostatin antiserum increased plasma GH level under basal conditions. The inhibitory effect of ether stress on plasma GH was noted again at the age 30 days and in adult animals. It is concluded that the hypothalamus of 3-day-old rats is able to release enough somatostatin to inhibit GH secretion after stress. At the period 12–18 days a phase of pituitary refractoriness was noted: ether stress as well as TRH injection (our previous observation) fail to affect plasma GH in female pups, probably due to high somatostatin secretion under basal conditions and (or) low capacity of pituitary to release GH. It is suggested that regulation of GH secretion is not mature until after the 21st day of lif