The effects of endothelin 1 on the internal maxillary artery blood flow, measured as an index of cerebral blood flow, were examined in six unanesthetized goats under control conditions, hypercapnia induced by inhalation of 10% CO2 in air, hypertension by intravenous infusion of norepinephrine, and hypotension by intravenous injection of diazoxide. Under control, administration of endothelin (0.01-0.3 nmol) into the internal maxillary artery produced dose-dependent sustained decreases in cerebral blood flow and increases in cerebrovascular resistance; higher doses (0.1 and 0.3 nmol) also caused hypertension and bradycardia. During hypercapnia or hypertension, endothelin did not significantly affect cerebral blood flow, and only higher doses (0.1 and/or 0.3 nmol) increased cerebrovascular resistance, but this was lower than under control. However, under hypotension endothelin evoked a higher reduction in cerebral blood flow and increment in cerebrovascular resistance, and systemic effects were also more marked than under control. Therefore endothelin is a potent cerebral vasoconstrictor, and this effect is very attenuated during hypercapnia and hypertension but is increased under hypotension.