Influence of infantile stimulation on the response to stress during preweaning development

New born rats were stimulated in infancy and the maturation of the adrenal response to stress or to exogenous adrenocorticotropic hormone (ACTH) was examined. Control animals (nonhandled) showed little or no response to ether and surgical trauma until about 18 days of age. In contrast, handled newborn rats had a significant elevation in plasma corticosterone concentration following stress introduced as early as 3 days of age, whereas at 21 days of age, nonhandled animals showed a significantly higher plasma corticosterone concentration than their handled counterparts. The response to ACTH was equivalent in both groups on each test day. Plasma steroid levels were elevated after ACTH injection at 3 days of age. This was followed by a period of marked suppression of the response to ACTH lasting until about 15 days of age in both groups.