The present experiments were carried out to investigate the endothelium dependence of the responses to acetylcholine (ACh), arachidonic acid, and histamine in monkey basilar arteries. ACh and arachidonic acid caused endothelium-dependent contraction (EC) in both monkey and canine basilar arteries. The endothelium-derived contracting factor (EDCF) was probably thromboxane A2 (TxA2), as the EDC was attenuated by a cyclooxygenase inhibitor, TxA2 synthetase inhibitors, and TxA2 antagonists. On the other hand, histamine caused endothelium-dependent relaxation (EDR) in monkey and EDC in canine basilar arteries. The EDR in monkey basilar arteries was attenuated by a nitric oxide synthase inhibitor. The EDR and EDC were antagonized by tripelennamine but not by cimetidine, indicating that they are mediated by H1-receptors. From these results, we suggest that in the monkey basilar artery, either there are two types of endothelium (an EDCF type for ACh and arachidonic acid and an EDRF type for histamine) or there is a single type of endothelium with two types of signalling processes (one for EDC and one for EDR).