Purpose.To investigate the effect of repeated testing on adaptation to a 2 &Dgr; vertical prism in subjects with normal binocular single vision.Methods.Subjects were required to have good visual acuity and normal binocular single vision. The residual deviation was measured after a 1-min period of adaptation to a 2 &Dgr; vertical prism using a modified Maddox Rod technique. This measurement was repeated 10 times, with a 5-min rest period between each trial.Results.For eight young adult subjects, the mean residual deviation at 1 min reduced with consecutive trials, and this effect was statistically significant (p < 0.0001). No significant correlation was present between the amplitude of vertical vergence and the increase in adaptation with repeated demand.Conclusions.A practice effect for vertical vergence adaptation appears to be present in young adult subjects with normal binocular single vision.