The effect of 24-h sleep deprivation by forced locomotion on plasma corticosterone was investigated in the rat. Corticosterone was slightly elevated after 21.5 h sleep deprivation, but did not differ from controls after a 2.5-h recovery period. An acute 20-min forced locomotion period caused a marked rise in plasma corticosterone. It is concluded that stress is not a major factor contributing to the massive effects of sleep deprivation on sleep parameters.