The study concerns the role of two combined factors-lipid composition of the microsomal membranes and the iron concentration in the incubation medium-in lipid peroxidation catalysed by paraquat (P(++)). Rats were subjected to diets containing 5% lipids composed of either tripalmitin (T), peanut oil/rapeseed oil (v/v) (C) or fish oil (F). The level of polyunsaturated fatty acids in the microsomal membranes was higher in C and F than in T. The level of vitamin E was lowest in F. The activity of the system 'Cyt P450-NADPH cyt c reductase' increased in the order T<C<F. The iron concentrations initiating a basal NADPH-dependent lipid peroxidation have been established. p(++) potentiates this peroxidation due to additional reduction of Fe(3+) by p(+.), rather than by O2(.-) as is usually thought to occur. The sensitivity of the membranes to the potentiating effect of P(+ +) is mainly determined by a high level of polyunsaturated fatty acids, but also by a low level of the antioxidant vitamin E.