The effects of practice on bilateral and unilateral reaction times (RTs) for thumb flexion tasks were investigated. Four female and 23 male students were divided into 4 groups who practiced a task under a single condition: unilateral (with either the right or left hand), bilateral, or control. Unilateral and bilateral groups practiced 20 trials 3 days a week for 6 weeks. For the left hand, the RT shortened following practice in the Unilateral-left and Bilateral groups; however, the interaction of the practice group by task was not significant. For the right hand, the RT shortened following practice, but neither the interactions of the practice group by task nor the practice group by measurement time were significant. The changes in the bilateral RTs with practice were significantly correlated with the change of the unilateral-left RT but were not significantly correlated with the change of the unilateral-right RT. As a result, no significant change in bilateral deficit was found. There was no lateral specificity in the effect of practice on RT.