The effects of Maharishi-4 (M-4) and Maharishi-5 (M-5) on microsomal lipid peroxidation were examined in vitro. Rat liver microsomes were incubated with an NADPH-generating system or with sodium ascorbate and an ADP-iron complex to stimulate enzymatic or nonenzymatic lipid peroxidation respectively. Alcoholic or aqueous extracts of M-4 or M-5, when added to these incubation systems, inhibited hepatic microsomal lipid peroxidation in a concentration-dependent manner. The aqueous extract of M-4 was the most effective antiperoxidant in these systems. A 10% (w/v) aqueous extract of M-4 inhibited ascorbate or NADPH-induced lipid peroxidation by approximately 50% when added at volumes of 8 microliters and 3.5 microliters respectively to the incubation mixtures (total incubation volume, 2 ml). These findings suggest that M-4 and M-5, by virtue of their antiperoxidant properties, may be useful in the treatment of free radical-linked drug toxicities and disease states.