The value of OD532 nm of malondialdehyed(MDA) formed by lipid peroxidation was used as an indirect index of reaction of free radicals. The roles of Hg2+, Mn2+, Ni2+, Cd2+, Cr3+, SeO3(2-) and water solution of As2O3 in vitro system of ascorbic acid producing free radicals were studied. The results indicated that Mn2+ and Cr3+ inhibited free radicals induced by ascorbic acid but on the contrary, Hg2+, Cd2+ and As2O3 showed an enhancing effect, and the roles of Ni2+, SeO3(2-) were ambiguous.