Endothelial thromboxane A2 (TXA2) in maintaining the resting tone and producing the contractile response to acetylcholine (ACh) and arachidonic acid was studied in canine cerebral artery. The spontaneous release of TXB2 from cerebral artery was about tenfold higher than that of coronary, mesenteric and saphenous arteries. The resting tone, the release of TXB2 and the contraction produced by arachidonic acid were decreased by the presence of cyclooxygenase inhibitor, TXA2 synthetase inhibitor, TXA2 antagonist and rubbing of the luminal side of preparations. The contraction produced by ACh was inhibited by the presence of the above inhibitors and rubbing of the preparations without decreasing the release of TXB2. These results suggest that the resting tone of canine cerebral artery and the contractile response to arachidonic acid are related to activation of TXA2 synthesis in the endothelium.