The effects of sodium iodate (SI), iodoacetic acid (IAA) and ethambutol (EB) on the electroretinogram (ERG) and the visual evoked potential (VEP) were examined in unrestrained rats. 1. A single intravenous dose of SI at 25 mg/kg caused depression of amplitudes of the ERG a-wave and oscillatory potentials 24 hrs after dosing. Following these changes, the amplitude of the ERG b-wave decreased. The depression of the VEP was observed in parallel with the depression in amplitude of the ERG. 2. A single intravenous dose of IAA, even at a dose of 60 mg/kg which induced death of the rats, did not cause any significant abnormality in the ERG and VEP. 3. Repeated subcutaneous dose of EB at 500 mg/kg/day depressed the amplitude of the P₁-N₁ wave and prolonged the peak latency of the P₁ and N₁ waves of the VEP without affecting the ERG after administration for 5 to 6 weeks. These abnormalities of the VEP appeared almost in parallel with ataxic gait. 4. Neither gross behavioral changes suggesting visual disturbances nor abnormal ocular fundus was revealed in any rat receiving SI or EB even when marked depression of the ERG and/or VEP was observed. 5. These results indicate that SI damages retinal function and EB does the conduction pathways from the retina to the visual cortex. In addition, the simultaneous recordings of both the ERG and VEP in unrestrained rats were found to be useful for evaluating the visual toxicity, and to furnish useful information on the site of toxic action of drugs.