The activity of transition metals under incubation conditions is of particular interest because these metal ions induced an insulin-like effect in fat cells which is reminiscent of that of hydrogen peroxide. It is possible that in one of its bound states nickel may stimulate hydrogen or lipid peroxide accumulation which in turn can account for this effect on adipocytes during in vitro incubation. In this study using a simple model system the role of metal ions on the formation and destruction of hydrogen peroxide is illustrated in medium used for mammalian adipocytes in-cubation. It is interesting to note that the only metal which generated hydrogen peroxide in the medium was copper when its concentration was 10 muM. The metals associated with destruction of hydrogen peroxide were cobalt, copper and manganese. Iron and nickel played no role at all.