The antioxidant and pro-oxidant properties of L-DOPA and dopamine were investigatedin vitro. Both compounds inhibited the peroxidation of ox-brain phospholipids, with IC50values of 8.5μM for dopamine and 450μM for L-DOPA. Dopamine and L-DOPA reacted with trichloromethyl peroxyl radicals (CC13O2) with rate constants of 2.1×107M1s-1and 1.3×107M-1s-1respectively. The effects of dopamine and L-DOPA on iron ion- dependent hydroxyl radical generation from H2O2were c:mplex. In general, low concentrations stimulated OH formation in the presence of ferric-EDTA and, in the case of L-DOPA, ferric-ADP and ferric citrate chelates. Both compounds also reacted with superoxide radical and hypochlor-ous acid. The products of the reaction with HOCI could still inhibit al-antiproteinase and appear to be 'long lived' chloramine-type oxidizing species. Our results suggest that L-DOPA and dopamine might have a complex mixture of pro- and anti- oxidant effects, which could contribute to tissue damage due to oxidative stress in Parkinson's disease and other neurological disorders.