Production of hydrogen peroxide in cells is a general phenomenon and, in particular, subcellular organelles such as peroxisomes [ 11, microsomes [ 1 ] and mitochondria [ 1,2] have been shown to be a source of HaOs. A number of tissues have been found to contain mitochondria, which are able to form HaO, under some conditions both in vitro and in vivo. In particular mitochondria from liver, heart and kidney have been shown to produce relatively high amounts of HsOa [3,9]. The role of HaOa in cells is still a matter of discussion, and in particular that of HaOa produced in mitochondria. Since the finding that heart mitochondria produce superoxide radicals and may dismutate them into HzOz [4], the possible production of Oaby other types of mitochondria must also be evaluated. Moreover the comparison between mitochondria coming from different tissues may cast some light on the possible role of Osand HaOa in cells. In the present study the production of HzOz in mitochondria from rat brain has been investigated. It was found that, contrary to heart and liver mitochondria, but similarly to Ehrlich ascites tumor cell mitochondria [5] they do not produce HsOa . Oaradicals are produced in all mitochondrial species tested including mitochondria from ascites tumor cells, except brain mitochondria.
