Psychological changes associated with ventrolateral thalamic lesions were assessed through standardized testing of parkinsonians undergoing initial left hemisphere, initial right hemisphere, and second side surgery for relief of tremor and rigidity. Immediate postoperative declines in cognitive and perceptual functions were found, with a return to preoperative status in follow-up testing. The concepts of physiological disruption and functional reintegration and compensation, respectively, were evoked to explain these findings. Lateralized differences were found with reference to tests involving use of verbal symbols in left hemisphere operates and second side operates, regardless of hemisphere, with worsening in performance immediately postoperatively. To a lesser degree, right brain operates were more affected in spatial-perceptual performance. Emphasis was placed on the relative roles of direction and arousal in behavior in considering the pattern of postoperative change.