The potential modifying effects of ethanol and paraquat on lesion development in livers and lungs of male F344 rats were studied. Animals were divided into diethylnitrosamine (DEN)-initiated and non-initiated groups, subgroups of each being exposed to 10 ml 20% ethanol/kg body weight, 2.5 or 10 mg paraquat/kg body weight or a combination of 10 ml 20% ethanol/kg body weight and 2.5 or 10 mg paraquat/kg body weight, given by intragastric intubation three times a week. Controls received 10 ml saline/kg body weight. All animals were subjected to two-thirds partial hepatectomy at the end of wk 3 and killed at the end of wk 8. All five rats receiving 10 mg paraquat/kg body weight without DEN-pretreatment died before termination of the experiment, but the additional ethanol treatment saved animals: only one of five rats died. Rats administered 2.5 or 10 mg paraquat/kg body weight demonstrated lung toxicity, as evidenced by fibrosis and hyperplasia, but not when simultaneously treated with ethanol in DEN-pretreated groups. In the liver, however, evaluation of glutathione S-transferase placental form (GST-P)-positive foci did not reveal any influence of the treatments on lesion development in DEN-initiated animals. It is concluded that ethanol decreases paraquat toxicity, and that neither agent, alone or in combination, exerts any hepatocarcinogenic modification potential.