1. This study was designed to investigate the effect of chronic exercise on exercise-induced changes in plasma concentrations of adrenocorticotrophic hormone (ACTH) and corticosterone in rats. Corticotrophin releasing factor (CRF) and ACTH were injected I.V. in order to assess the responsiveness of the pituitary and adrenal glands after chronic exercise. 2. The concentrations of ACTH and corticosterone in the plasma increased significantly after acute exercise in both the control and the exercised groups but both responses were significantly smaller in the exercised group. 3. The ACTH response to I.V. CRF was also significantly smaller in the exercised rats. However, both groups of animals showed similar increases in plasma corticosterone levels after the administration of exogenous ACTH. 4. The ACTH response to CRF attenuated by repeated administration of CRF. 5. These results suggest that attenuated ACTH and corticosterone responses to acute exercise after chronic exercise result from reduced responsiveness of the pituitary gland to CRF.