1. We have characterized the constrictor endothelin receptors present in human isolated blood vessels using ETA and ETB selective agonists and antagonists. 2. Monophasic dose-response curves were obtained for ET-1 with EC50 values of 6.8 nM in coronary artery, 3.9 nM in internal mammary artery, 17.4 nM in pulmonary artery, 14.5 nM in aorta and 3.2 nM in saphenous vein. In coronary artery, ET-2 was equipotent with ET-1 with an EC50 value of 5.7 nM. The non-selective peptide, sarafotoxin 6b, was 2-3 times less potent than ET-1 but the maximum responses to these two were comparable. 3. In each vessel ET-3 was much less active than ET-1. No response was obtained to ET-3 in aorta and pulmonary artery or in up to 50% of coronary artery, mammary artery and saphenous vein preparations. In those preparations that did respond, dose-response curves were incomplete at 300 nM. Variable contractions were also obtained with the ETB-selective agonist, sarafotoxin 6c (S6c). Where responses were detected, although S6c was more potent than ET-1 (EC50 values of 0.6-1.2 nM), the maximum response produced was always less than 20% of that to ET-1. 4. The synthetic ETB agonists, BQ3020 and [1,3,11,15Ala]-ET-1, were without effect in any of the five blood vessels at concentrations up to 3 microM. 5. ET-1-induced vasoconstriction was blocked by the ETA-selective antagonists, BQ123 and FR139317. Schild-derived pA2 values were 7.0, 7.4 and 6.9 for BQ123 and 7.6, 7.9 and 7.3 for FR139317 in coronary artery, mammary artery and saphenous vein, respectively, consistent with antagonism of ETA receptors. Slopes of the Schild regressions were not significantly different from one. Comparable values of pA2 were estimated for 3ftM BQ123 in aorta (7.4+/-0.5) and pulmonary artery (6.9) from the Gaddum-Schild equation.6. In conclusion we have shown that in human isolated blood vessels, ET-1 is more potent than ET-3 suggesting the presence of vasoconstrictor ETA receptors. This is supported by the lack of effect of the ETB agonists, BQ3020 and [1,3,1,1,15Ala]-ET-l and the ability of the ETA antagonists, BQ123 andFR139317 to block ET-1 responses. Some preparations did contract in response to low concentrations of the ETB-selective sarafotoxin 6c but responses were variable and the maximum was always much less than that to ET-1 in the same preparations. Therefore although constrictor ETB receptors were present on the smooth muscle of human blood vessels, vasoconstriction elicited by the endothelin peptides in vitro is via ETA receptor activation.