The kinetics of hepatic microsomal ethanol oxidation (MEOS) was studied in microsomal suspension from rat liver. The estimated maximum elimination rate was 117 nmol acetaldehyde/ min/g liver and the half-saturation concentration 12.7 mmol ethanol/l. The MEOS activity was not influenced by the presence of antipyrine 0.1 mmol/l and 1.0 mmol/l in the microsomal incubations.