OBJECTIVE (+/-)-SB 209670, a potent nonpeptide endothelin (ET) receptor antagonist, was used to investigate the potential role of ET in cerebral vasospasm associated with subarachnoid hemorrhage. METHODS The effects of (+/-)-SB 209670 were evaluated in isolated segments of canine posterior cerebral arteries in vitro, vascular smooth muscle cells in culture, and in the canine two-hemorrhage model of delayed cerebral vasospasm in vivo. RESULTS In the canine basilar and anterior spinal arteries, (+/-)-SB 209670 caused a dose-related inhibition of contractile responses mediated by ET (KB = 4.6 nmol/L and apparent KB = 2.7 nmol/L, respectively). The effects of (+/-)-SB 209670 were mediated by inhibition of ETA receptors since the ETB selective agonist sarafotoxin 6c did not contract these posterior cerebral vessels. (+/-)-SB 209670 also produced a concentration-dependent inhibition (IC50 = 1 nmol/L) of the mitogenic response induced by ET-1 in vascular smooth muscle cell culture. In the canine model of delayed cerebral vasospasm, animals received intracisternal vehicle (saline) or (+/-)-SB 209670 (360 +/- 10 micrograms/d) via osmotic minipump for 7 days. On day 7, the cross-sectional areas in the (+/-)-SB 209670 group were significantly greater than those in the vehicle group in both the basilar artery (68% versus 27%) and anterior spinal artery (78% versus 38%). No differences in blood pressure or heart rate were noted in the two groups, and the vasospasm in the vehicle group did not differ from that of historic controls in this model. CONCLUSIONS The results suggest that ET plays a significant role in the development of delayed cerebral vasospasm via an interaction with ETA receptors. Furthermore, ETA receptor antagonists may represent a novel therapeutic approach to the treatment of subarachnoid hemorrhage.