['To examine the effects of hypertension on systemic and regional haemodynamic responses to endothelin-1.', 'Comparison of responses between age-matched control and hypertensive rabbits (two-kidney, two wrapped), and between spontaneously hypertensive rats (SHR) and control Wistar-Kyoto rats.', 'Arterial pressure, heart rate and blood flow responses to 0.2 nmol/kg intravenous endothelin-1 were measured in conscious animals. Blood flow was measured by pulsed ultrasound Doppler in the ascending aorta, distal abdominal aorta, left renal artery and superior mesenteric artery.', 'Endothelin-1 produced qualitatively similar effects in the hypertensive and control animals. In the systemic circulation, brief initial vasodilation preceded sustained vasoconstriction. In the hindlimb, marked vasodilation preceded relatively minor vasoconstriction, and profound vasoconstriction occurred in the renal and mesenteric vascular beds. In the rats but not the rabbits, fleeting vasodilation preceded the renal and mesenteric vasoconstriction. Significant differences between hypertensive and control animals were: accentuation of the pressor effect and heart rate responses in hypertensive animals of both species, and accentuation of hindlimb vasodilation in hypertensive rabbits but not SHR; and attenuation of the depressor effect in SHR but not hypertensive rabbits, attenuation of the mesenteric vasoconstriction in both hypertensive rabbits and rats, and attenuation of renal vasoconstriction in SHR.'] Does experimental hypertension produce diverse changes in the regional vascular responses to endothelin-1 in the rabbit and the rat?