The brain dopaminergic system is a critical modulator of basal ganglia function and plasticity. To investigate the contribution of the dopamine D1 receptor to this modulation, we have used gene targeting technology to generate D1 receptor mutant mice. Histological analyses suggested that there are no major changes in general anatomy of the mutant mouse brains, but indicated that the expression of dynorphin is greatly reduced in the striatum and related regions of the basal ganglia. The mutant mice do not respond to the stimulant and suppressive effects of D1 receptor agonists and antagonists, respectively, and they exhibit locomotor hyperactivity. These results suggest that the D1 receptor regulates the neurochemical architecture of the striatum and is critical for the normal expression of motor activity.