Plasma corticosterone levels were measured fluorometrically in male DBA/2J and C57BL/6J mice at various ages, under control conditions and after exposure to an open-field activity situation. Additionally, mice of the two strains were injected with varying doses of ACTH in order to observe the responsiveness of the aging adrenal. Results indicate a progressive and declining adrenal response to ACTH which is significantly more pronounced in DBA/2J than C57BL/6J mice. Exposure to the open-field situation revealed significantly higher reactivity of C57BL/6J than DBA/2J mice when compared to basal conditions. A number of hypotheses are offered for explaining the differences with advancing age in the two strains.