1. A microsomal lipid peroxidation enzyme system in rainbow trout white skeletal muscle was identified and characterized. 2. This enzyme system is very similar to that found in other fish such as red hake, winter flounder and herring. 3. NADH-cytochrome b5 reductase is suggested to be the enzyme involved, because of the specific requirement for NADH as the electron donor, the inhibition of enzyme activity by p-hydroxy-mercuribenzoate and the involvement of chelated non-heme iron. 4. The activating and inhibitory effects of EDTA and KCN on lipid peroxidation enzyme system activity at different concentrations are discussed extensively in terms of the possible initiation reaction mechanism.