Checkerboard pattern reversal visual evoked potentials were studied in 18 patients with Parkinson's disease and in 20 normal controls without ocular diseases. 1. The interocular differences of Pl latencies and Nl-Pl amplitudes were significantly larger in the Parkinsonian group than in the normal control. 2. The absolute Pl latency was longer and the absolute Nl-Pl amplitude was smaller in the Parkinsonism group before dopaminergic therapy than in the normal control. 3. The absolute Pl latency was also longer in the Parkinsonian group after a peiod of levodopa treatment than in the normal control, but there was no significant difference of absolute Nl-Pl latency between them. From these findings, authors confirmed the suggestion that at least part of the delay of the Pl latency occurs at prechiasmal level of the visual system in Parkinson's desease.