Ethane evolution was measured in rats breathing pure oxygen. Animals injected i.p. with a lethal dose of paraquat (50 mg/kg) developed signs of pulmonary insufficiency within 3 hours and died within 24 hours. Ethane evolution, a parameter of lipid peroxidation in vivo, was increased over control levels only by 26% after 4 hours. It is concluded that this increase is too small to support the theory that lipid peroxidation is the biochemical mechanism of paraquat toxicity.