The dorsal striatum in the basal ganglia circuitry is a principal structure that mediates the acquisition and performance of instrumental learning. The projections from the dorsal striatum are composed of two subpopulations of medium spiny neurons that constitute the direct and indirect pathways. The mechanism by which these striatal projections control the learning processes of instrumental actions remains unknown. We addressed the behavioral role of the striatal direct (striatonigral) pathway in the performance of visual discrimination. Immunotoxin targeting of the striatal neuronal type containing dopamine D1 receptor in mice resulted in a moderate level of elimination of the striatonigral pathway. Targeting of the neural pathway from the whole region of the dorsal striatum lengthened the response time but did not affect the accuracy of response selection in a two-choice reaction time task dependent on light stimulus. This lengthened motor response was induced early in the test sessions and was gradually restored to normal levels during repetitive sessions. In addition, subregion-specific pathway targeting revealed that the delay in learned motor response was generated by the elimination of the striatonigral pathway arising from the dorsomedial striatum but not from the dorsolateral striatum. Our findings indicate that the striatonigral pathway, in particular from the dorsomedial striatum, contributes to the regulation of response time in the execution of visual discrimination. The restoration of motor response deficits during repetitive sessions suggests the presence of a mechanism by which the response facilitation is acquired through continuation of learning despite the removal of the striatonigral pathway.