1. The vasoconstrictor effects of endothelin-1 were studied in perfused mesenteric vascular beds (MVB) and aortic rings of 14-16 week-old spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). 2. Reactivity to endothelin-1 was increased in MVBs of SHR, as indicated by the maximum perfusion pressure obtained (264 +/- 8 and 141 +/- 9 mmHg respectively) (P less than 0.001), whereas sensitivity was not significantly different between the two strains (EC50 171 +/- 21 and 102 +/- 19, respectively). 3. In aortic rings, in contrast, reactivity to endothelin-1 was reduced in SHR as compared to WKY, whereas sensitivity was similar (EC50 0.78 +/- 0.08 and 0.87 +/- 0.09 nM). 4. As with endothelin-1, reactivity to noradrenaline and potassium chloride was increased in MVBs, but not in aortic rings of SHR. Endothelin-1 was 30 times more potent than noradrenaline in MVBs of SHR, and 15 times more potent than noradrenaline in aortic rings. 5. In both strains, nifedipine and nitrendipine almost completely blocked potassium-induced contractions in MVB and aortic rings, respectively, whereas contractions induced by endothelin-1 or noradrenaline were only partially inhibited. 6. It is concluded that calcium influx via the voltage-operated calcium channel is only partially responsible for the vasoconstrictor action of endothelin-1 in MVBs and aortic rings of SHR and WKY rats. The increased reactivity of the MVB of SHR to endothelin-1 at this stage of the hypertensive process is most likely to be the result of a change in vascular structure rather than due to a primary hypertensive mechanism.