peroxynitrite　is　a　toxic　biological　oxidant　which　has　been　implicated　in　many　pathophysiological　conditions.　Metalloporphyrins　can　catalyze　it　decomposition　to　protect　the　cells.　A　cobalt　water-soluble　porphyrin　Co(II)TPPOC3PY　was　synthesized　and　its　ability　to　catalyze　peroxynitrite　decomposition　was　studied　by　stopped-flow　spectrophotometer.　The　catalytic　activity　of　Co(II)TPPOC3Py　is　comparable　to　iron　and　manganese　porphyrins　which　have　been　investigated　before.　The　catalytic　mechanism　is　related　to　the　intermediate　formed　from　the　reaction　of　Co(II)TPPOC3Py　and　peroxynitrite,　and　the　catalytic　ability　depends　on　whether　the　intermediate　is　reduced　easily.