The non-toxic and water soluble dihydroquinoline type antioxidants: CH 402 (Na-2,2-dimethyl-1,2-dihydroquinoline-4-yl methane sulphonate) and MTDQ-DA (6,6-methylene bis 2,2-dimethyl-4-methane sulphonic acid: Na-1,2-dihydroquinoline) were studied in various in vitro tests in which oxygen free radicals were generated. Both compounds were shown to scavenge superoxide radical anions O2- produced in aqueous solution by pulse radiolysis with rate constants k (O2- + MTDQ-DA) = 4.10(8) dm3 mol-1 s-1 and k (O2- + CH 402) = 1.5.10(7) dm3 mol-1 s-1. CH 402 and MTDQ-DA reduced the H2O2 produced in the glucose-glucose oxidase reaction, which was detected by the luminol + hemin reaction with a chemiluminometric method. The dihydroquinoline type substrates inhibited the NADPH-induced and Fe3+ - stimulated lipid peroxidation and the ascorbic acid-induced non-enzymatic peroxidation pathways in microsomal fractions of rat and mouse liver.