It is generally assumed that the coupling of dopamine D1 receptors to adenylyl cyclase is mediated by the stimulatory GTP-binding protein G(s). However, the striatum contains little G(s)alpha subunit, whereas it expresses high levels of G(olf)alpha, a close relative of G(s)alpha that is also expressed in olfactory receptor neurons. We used G(olf)alpha knockout mice to examine the functional coupling of D1 receptors. We found that these mice showed no hyperlocomotor response to either the D1 agonist SKF-81297 or the psychostimulant cocaine. Moreover, G(olf)alpha knockout mice did not display cocaine-induced c-fos expression in the striatum. Finally, in the absence of G(olf)alpha, striatal D1 receptors have a decreased affinity for dopamine. Thus coupling to G(olf)alpha appears to mediate D1 signaling in the striatum.