The possibility that lipid peroxidation is involved in valproic acid (VPA) hepatotoxicity was explored by testing the ability of the free-radical scavengers alpha-tocopherol (vitamin E) and N,N'-diphenyl-p-phenylenediamine (DPPD) to protect against VPA toxicity. Rat hepatocyte cultures were treated with toxic doses of VPA, in conjunction with varying doses of vitamin E and DPPD. Lactate dehydrogenase (LDH) release into the culture media was used to calculate an LDH index as a measure of toxicity. Vitamin E afforded increasing protection against VPA toxicity at concentrations of 1.0 to 4.0 microM but then leveled off and did not give complete protection at concentrations up to 8.0 microM. No protection was seen at less than 1.0 microM. DPPD showed increasing protection from 0.05 to 0.50 microM, with complete protection at the highest concentration. These data indicate that VPA toxicity can be prevented by simultaneous administration of free-radical scavengers and support the concept that VPA hepatotoxicity is due to lipid peroxidation.