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The development of the pituitary-adrenocortical stress response was studied in CBA/J \m=x\ DBA/2J hybrid mice. On the basis of the plasma corticosterone response 15 min after a subcutaneous injection of histamine dihydrochloride (50 mg/kg), the first three neonatal weeks could be divided into stress-nonresponsive (3\ p=n-\ 211 days) and stress-responsive 16\p=n-\21days) periods. During the former period, corticosterone levels in the brains of the non-stressed control mice were 63 % higher than those of comparable mice during the latter period. Histamine stress significantly increased corticosterone concentrations in the brain during both these periods, but the increase was much greater (88 %) during the stress-responsive period than during the stress-nonresponsive period (29 %).