When porcine adrenocortical microsomes were treated with Fe 2+, enhanced production of malondialdehyde was observed as a result of membrane lipid peroxidation. By treatment of the microsomes with Fe 2+, the activity of Δ 5-3β-hydroxysteroid dehydrogenase coupled with Δ 5-Δ 4 isomerase, concentration of cytochrome P-450 and the activity of the cytochrome-involving enzyme systems such as 17α- and 21-hydroxylases were significantly reduced. 17α-Hydroxylase activity was more effectively decreased by Fe 2+ than that of 21-hydroxylase. On the other hand, activity of NADH- and NADPH-cytochrome c reductases remained unchanged or somewhat increased. Both the induction of lipid peroxidation and the decrease of the enzyme activities were prevented by α-tocopherol and N,N′-diphenyl- p-phenylenediamine.