The actions and interactions between ascorbate, NADPH, and ferrous or ferric ions on lipid peroxidation (LP) were investigated in adrenal and testicular microsomes from male English-short-hair- guinea-pigs. The addition of either ascorbate or ferrous ion (Fe2+) to the incubation flasks stimulated LP in both adrenal and testicular microsomes, with the amount of LP being dependent on the concentration of Fe2+ or ascorbate in the reaction mixture. Similar time courses for Fe2+ and ascorbate induced LP were found in both adrenal and testicular microsomes. In both microsome samples, Fe2+ was a much more potent stimulus for LP than was ferric ion exposure. LP was not stimulated by NADPH alone in either microsome sample. However, the addition of low levels of Fe2+ allowed NADPH to produce concentration dependent increases in LP in adrenal microsomes, suggesting an enzymatic process. In adrenal microsomes with low Fe2+ concentrations, NADPH inhibited LP in a concentration dependent manner which was fully demonstrable in heat treated microsomes, indicating a nonenzymatic effect. The authors suggest that LP could contribute to the control of steroid hormone synthesis and release and that various endogenous substances may aid in controlling adrenal and testicular LP, with the nature of that regulation differing between tissues.