Low physical activity (PA) levels are associated with many chronic diseases and place a massive burden on the health care system. The extrinsic environment has been suggested to regulate activity levels, but an increasing prevalence of physical inactivity in developed societies suggests the presence of other regulating factors. The sex steroids affect PA levels, but the regulating mechanisms are not known. The purpose of this study was to evaluate the interrelationship of sex steroids and PA in mice and the involvement of dopamine 1 receptors (Drd1) in sex steroid regulation of PA. First, sex steroid levels were manipulated in male and female mice by gonadectomy and replacement via silastic implants. Wheel running decreased in both sexes after removal of the gonads. Testosterone administration to gonadectomized mice completely reversed the PA deficit while administration of 17β-estradiol recovered 50% of the deficit. Next, the aromatase complex was pharmacologically inhibited. Wheel running levels remained unchanged in normal, orchidectomized, and steroid replaced male mice suggesting the presence of an androgen responsive PA regulating mechanism. Finally, PA levels were observed after orchidectomy in mice receiving either testosterone or a Drd1 antagonist. PA levels were elevated by testosterone, but were unaffected by the antagonist. The levels of Drd1 mRNA in the nucleus acumbens and striatum were evaluated after administration of these two compounds. Drd1 mRNA expression was not different between treatments or compared to control mice. These results suggest that PA is regulated by the estrogens and androgens, but the influence is not regulated by Drd1.