Administering corticosterone to rat pups for 4 days early in life can lead to improved maze performance and altered stress response compared to pups treated later or not at all. Pups exposed to corticosterone had higher levels of the hormone initially, but lower levels at weaning. Early treatment resulted in faster learning in a maze task and higher stress hormone release in response to stress in female pups. There was no difference in corticosteroid receptor levels in the hippocampus among the groups. The timing of corticosterone treatment seems to be crucial, with early exposure being more effective than later exposure.