The effects of acetaldehyde and ethanol treatments on hepatic lipid peroxidation are compared in rats after the administration of toxically corresponding doses. Wistar rats were given a single dose of acetaldehyde (0.5 g/kg) or ethanol (5 g/kg) intraperitoneally. The animals were killed 1/2, 1, 2, 4, 6, 9, 12 and 24 h after treatments. The results indicate that acetaldehyde, like ethanol, stimulates hepatic lipid peroxidation and also selectively affects the hepatic mitochondria.