Effects of neonatal treatment with cortisol on the mortality, growth rate, the weight of thymus, spleen and adrenal gland, blood lymphocyte counts and stress responses were examined in the rat. In rats treated with 1 mg cortisol the survival rate was only 13 percent, and in those given 0.75 mg the rate was 47 percent. Large doses of cortisol produced "wasting syndrome" and the animals died during a period from 5 to 13 days after the injection. Over the first week the body weight did not increase in rats injected with cortisol in doses of 1 and 0.75 mg. A marked atrophy of the thymus and a great reduction of the blood lymphocyte counts were observed in rats treated with large doses of cortisol. The development of spleen was temporarily inhibited. The adrenal gland was remarkably atrophied: the gland weight at 21 days of age was similar to that at birth and only 22 percent of the control value. In rats injected with 0.5 mg cortisol plasma corticosterone response to ether inhalation was not different from that of normal controls at 21 days of age. However, rats treated with 0.75 and 1 mg cortisol did not show plasma corticosterone response on exposure to a strange environment. The effect of several kinds of corticoids was also examined. Dexamethasone and betamethasone caused a marked atrophy of the thymus and spleen. Cortisol and deoxycorticosterone produced a large decrease in the adrenal corticosterone content. The effect of exogenous corticosterone was less in extent as compared with that of cortisol.