Dopamine D3 receptors are present in the nucleus accumbens and in lobules 9 and 10 of the cerebellum. Their function is not fully understood. In the present study, the involvement of D3 receptors in locomotor activity and sleep in the two structures was investigated. The method used was the measure of locomotor activity in an open field and the measure of sleep and sleep/waking stages by electroencephalographic recordings in the rat. Dopamine receptor agonists (7-OH-DPAT, quinelorane and SKF38393) and antagonists (amisulpride, nafadotride and haloperidol) were microinjected into the two structures. Concerning locomotor activity, the results show that D3 receptor stimulation, as well as concomitant stimulation of D1 and D3 receptors, have opposite effects in the cerebellum and in the core of nucleus accumbens, with an inhibition of locomotion in the cerebellum and a stimulation of locomotion in the nucleus accumbens. D1 and D3 stimulation have mutual potentiating locomotor effects. The results also show that in the cerebellum, the effects are selective to lobules 9 and 10, and are not observed with microinjections into lobule 8. Concerning sleep, the effects of agonists and antagonists show a similar tendency in the cerebellum and nucleus accumbens, with a somnogenic effect of D2 blockage, an awakening effect of at least one of the D2/D3 agonists, and a dose-dependent awakening effect of the D2/D3 antagonists. In conclusion, this study suggests that dopamine D3 receptors in the cerebellum and nucleus accumbens are involved in several aspects of the regulation of locomotor activity and sleep.