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The relationship between circadian changes in spontaneous motor activity in rats and dopamine (DA) neurotransmission in the dorsal or ventral striatum was assessed with on-line in vivo microdialysis. The concentration of DA and DA metabolites in the dorsolateral caudate nucleus increased significantly at night. In contrast, DA in the nucleus accumbens did not change significantly across the light–dark cycle. The concentration of DA metabolites in the nucleus accumbens did show circadian variation, however, which was comparable with that seen in the dorsolateral caudate nucleus. Although there was a significant positive correlation between the concentration of DA in both the dorsal and ventral striatum and spontaneous nocturnal motor activity, the relationship was very weak, especially for the accumbens. This suggests that regulating the level of spontaneous motor activity per se is not a primary function of the mesostriatal DA system.