The effect of voluntary wheel-turning activity on sleep was investigated in the rat. In addition to scoring the conventional sleep states, a frequency analysis of the telemetrically recorded EEG-signal was performed to define a slow wave sleep (SWS) fraction of non-REM sleep (NREMS). The animals had access to the running-wheel during the first 3.5 days and the last 2.5 days of the 8-day recording period, but were prevented from entering the wheel in the 2 intervening days. Wheel-running (average distance run per day: 3015 m) occurred mainly during the dark-phase. The level of SWS was not significantly different for periods with access and without access to the wheel. The dark-time level of SWS relative to NREMS showed a trend towards higher values in the wheel-access period. The dark-time level of REM sleep was negatively correlated with the distance run in the wheel, and was lower in the access nights than in the no-access nights. Although locomotor activity produced some shifts in the sleep state distribution in the dark-phase, it had practically no effect on the rat's main daily sleep period in the light-phase.