A procedure for recording the electroretinogram (ERG) in rats with a contact lens-type electrode was developed in order to examine visual toxicity over-time. Rats received a single intravenous injection of sodium iodate (SI), a retinotoxic compound, via the tail vein at a dose of 12.5, 20, 25 or 50 mg/kg, and the ERG was recorded for 10 days after dosing. Histopathologic examination of the retinas was then conducted. 1. The rats were anesthetized with ketamine hydrochloride (50 mg/kg, i.m.) after 90 to 120 min of dark-adaptation. Thirty-two responses to repetitive 1.2 joule light stimuli at 0.5 Hz interstimulus intervals were averaged by a microcomputer. Under these conditions, stable ERG a-wave, b-wave and oscillatory potentials could be recorded for 10 days. 2. At 12.5 mg/kg of SI, no treatment-related abnormalities were observed on the ERG. Doses of 20 mg/kg or more of SI caused depression of the amplitudes of the ERG a-wave and oscillatory potentials 2 hrs or 1 day after dosing. Following these changes, the amplitude of the ERG b-wave decreased 1 or 2 days after dosing. 3. Upon histopathologic examination of the retina, folding of the outer nuclear layer, disarrangement of the rods and cones and swelling and decrease of the pigment epithelial cells were observed at 20 mg/kg or more. The severity of the retinal lesions correlated well with the changes in the ERG. 4. Using this recording technique, it was confirmed that a stable ERG could be recorded repeatedly in rats, and the effects of SI on the ERG could be detected. Furthermore, histopathologic examination revealed that the severity of the retinal lesions correlated well with the changes in the ERG. These results indicate that the ERG recording technique employed in this study is useful for evaluating retinal toxicity in rats.