The roles of PGI2 and TXA2 in recurring reduction of carotid artery and cerebral blood flow induced by partial constriction of the common carotid artery and cerebral blood flow induced by partial constriction of the common carotid artery were examined in anesthetized dogs. The recurring reduction was eliminated by OKY 046 and 1580 which inhibit TX synthetase, acetylsalicylic acid which inhibits cyclo-oxygenase and lipoxygenase, PGI2 and by papaverine which enhances PGI synthesis. But the recurring reduction was not eliminated by phentolamine. The recurring reduction was induced by epinephrine which activates phospholipase A2 and cyclo-oxygenase and causes platelet aggregation. It was also induced by tranylcypromine which inhibits PGE2 synthetase and, although infrequently, by TXA2. The recurring reduction was also induced by indomethacin that inhibits cyclo-oxygenase. The indomethacin-induced recurring reduction, however, was eliminated not by OKY 046 and 1580 but by PGI2. It is suggested that TXA2 acted as an inducer and PGI2 as an inhibitor in the recurring reduction of carotid artery and cerebral blood flow.