Circadian rhythm alterations are related to the onset and severity of various diseases. The expression of the dopamine receptor D3 (DRD3) is regulated by clock genes, and DRD3 functional abnormalities are linked to various neurological diseases. However, the relationship between DRD3 function and circadian machinery is unclear. Here, we demonstrate the influence of DRD3 on the circadian machinery. Although the expression of DRD3 in mouse suprachiasmatic nucleus (SCN) did not show a circadian rhythm, the expression of Per1 mRNA was altered in the SCN of Drd3 knockout (Drd3