Visual performance improves with practice (perceptual learning). In this study we sought to determine whether or not adult monkeys reared with early abnormal visual experience improve their stereo-acuity by extensive psychophysical training and testing, and if so, whether alterations of neuronal responses in the primary visual cortex (V1) and/or visual area 2 (V2) are involved in such improvement. Strabismus was optically simulated in 5 macaque monkeys using a prism-rearing procedure between 4 and 14 weeks of age. Around 2 years of age, three of the strabismic monkeys ("trained" monkeys) were tested for their spatial contrast sensitivity and stereoacuity. Two other strabismic monkeys received no training or testing ("untrained" monkeys). Microelectrode experiments were conducted around 4 years of age. All three trained monkeys showed substantial improvement in stereoacuity. However, final stereothresholds were still about 10-20 times worse than those in normal monkeys. In V1 disparity sensitivity was drastically reduced and behavioral training had no obvious effect in both the trained and untrained strabismic monkeys. In V2, disparity sensitivity was significantly better in the trained monkeys than in untrained monkeys. However, this difference in disparity sensitivity was not sufficient to account for the total magnitude of perceptual improvement. These data suggest that the observed improvement in stereoacuity of the trained strabismic monkeys may have resulted from better retention of disparity sensitivity in V2 and from "learning" by upstream neurons to more efficiently attend to residual local disparity information in V1 and/or V2.