SUMMARY Rats were undernourished during the foetal and suckling periods by limiting maternal food intake. Animals were weaned to an unrestricted diet at 25 days of age. Before weaning, the mean plasma corticosteroid concentration in undernourished animals was 39 μg/100 ml compared with 21 μg/100 ml in well-fed controls. At 15–17 weeks of age previously undernourished and control animals did not differ in basal plasma corticosteroid concentration. After stressful stimulation, however, levels were lower in animals subjected to early undernutrition than in controls. It is suggested that nutritional deprivation in infancy resulting in increased plasma corticosteroid levels at that time, permanently modifies the adrenocortical response to stress.