The influence of exercise on the endogenous levels of both β-endorphin (BE) and corticosteroid (CS) in the hypothalamus and plasma was examined in rats. F344 male rats, five weeks of age, were divided into three groups: involuntary exercised (IE), non-exercised (NE) and control (C) groups. Rats in the IE group were exercised every day for one hour at the speed of 20 m mm n for 7 to 28 days. After training, the hypothalamus and plasma were obtained from rats. BE levels in water soluble extracts of hypothalamus were examined by rat BE ELISA test kits; and CS contents in plasma was examined by RIA. The amount of BE in the hypothalamus obtained from rats in the IE group was much higher than that in the NE and C groups. Although there was no significant difference between rats exercised for 7, 14, and 21 days, BE levels in the hypothalamus from rats exercised for 28 days was significantly higher than for 7 days. The amount of BE in the hypothalamus of NE rats was nearly identical to that of C rats. On the other hand, there were no significant changes in plasma CS levels. These results may suggest that exercise, especially running, causes on increase in BE levels in the hypothalamus.