The effect of various oxidants on bovine aortic prostacyclin synthase was tested with 14C-labelled prostaglandin endoperoxide as substrate. No sensitivity against hydrogen peroxide, superoxide or hydroxyl radicals was observed but hypochlorite inhibited with an IC50 value of 7 microM. Among the reactive nitrogen species nitric oxide and nitrogen dioxide radicals were ineffective, but peroxynitrite irreversibly blocked prostacyclin biosynthesis with an IC50 value of 50 nM. Peroxynitrite acted within seconds whereas hypochlorite required up to 30 min for completion. Simultaneous generation of nitric oxide and superoxide also caused inhibition which suggested that under pathological conditions like ischemia-reperfusion not only the vasodilatory effects of nitric oxide but also those of prostacyclin could be eliminated.