Dopamine D1 receptors play an important role in the effects of cocaine. Here we investigated the role of neurons, which express these receptors (D1-neurons) in the acute locomotor effects of cocaine and the locomotor sensitization observed after a second injection of this drug, using the previously established two-injection protocol of sensitization. We inhibited D1-neurons using double transgenic mice conditionally expressing the inhibitory Gi-coupled designer receptor exclusively activated by designer drugs (Gi-DREADD) in D1-neurons. Chemogenetic inhibition of D1-neurons by a low dose of clozapine (0.1 mg/kg) decreased the cocaine-induced expression of Fos in striatal neurons. It diminished the basal locomotor activity and acute hyper-locomotion induced by cocaine (20 mg/kg). Clozapine 0.1 mg/kg had no effect by itself and did not alter cocaine effects in wild type mice. Inhibition of D1-neurons during the first cocaine administration prevented the sensitization of the locomotor response in response to a second cocaine administration ten days later. On day 11, inhibition of D1-neurons by clozapine stimulation of Gi-DREADD blocked cocaine-induced locomotion including in sensitized mice, whereas on day 12, in the absence of clozapine and D1-neurons inhibition, all mice displayed a sensitized response to cocaine. These results show that chemogenetic inhibition of D1-neurons decreases spontaneous and cocaine-induced locomotor activity. It prevents the induction and expression of sensitization in a two-injection protocol of sensitization but does not alter established sensitization. Our study further supports the central role of D1-neurons in mediating the acute locomotor effects of cocaine and its sensitization.