The relative roles of endothelin ET(A) and ET(B) receptor activation in cerebral vasospasm following subarachnoid hemorrhage were investigated in the rabbit. The endothelin ET(A) receptor antagonist, BQ610 (1 mu M; homopiperidinyl-CO-Leu-D-Trp(CHO)-D-Trp-OH), and the endothelin ET(A)/ET(B) receptor antagonist, PD145065 (1 mu M; Ac-D-Bhg-L-Leu-L-Asp-L-Ile-L-Ile-L-Trp), relaxed the vasospastic basilar artery in situ by 45% and 87%, respectively. These results suggest that subarachnoid hemorrhage-induced vasospasm of the rabbit basilar artery is due to activation of both endothelin ET(A) and ET(B) receptors.