Rats treated with cortisol, 0.5 mg one day after birth, show a decrease in body and organ weight and a decrease in pituitary TSH and GH when compared to controls together with a lack of response of TSH to TRH. These alterations have been previously found in animals treated with large doses of thyroxine, neo-T4 syndrome, in both populations a brain underdevelopment has been observed. Plasma insulin in cortisol-treated animals was higher (P less than 0.01) than in the controls at 8 and 12 days of life and lower at 22 days (P less than 0.001). Plasma ketone bodies were increased significantly at 8 but not at 12 and 22 days of life in the cortisol treated animals. Liver glycogen was higher than in controls at 2, 4, 6 days of life and lower at 8, 12 and 22 days. Blood glucose was higher than in controls at 2, 4 days and lower at 6, 8, 12 and 22 days of life. The timing of metabolic events is important for the adaptation and development of the post-natal animal. The perturbances in carbohydrate metabolism in suckling rats were also previously found in neo-T4 rats during the period when the brain growth spurt and energy need are the greatest; but in the rats treated with cortisol the timing of the alterations was different. This different timing of the metabolic perturbances could correlate with a different maturation in cerebral tissue found in histological studies in the two populations.