1. The role of the ET(B) receptor in human arteries has not been well studied because of the lack of specific ET(B) receptor antagonists. In the present studies the specific ET(B) receptor antagonist BQ-788 and the specific ET(B) agonist IRL-1620 were used to characterize the function of the ET(B) receptor in human radial arteries and internal mammary arteries. 2. The results showed that the ET(B) antagonist BQ-788 significantly inhibited endothelin-1-induced contraction in internal mammary arteries, but not in radial arteries. In internal mammary arteries, BQ-788 at a concentration of 10 micromol/l shifted the endothelin-1-induced concentration-dependent curve to the right by one order. By comparison, the ET(A) receptor antagonist BQ-610 at 1 micromol/l produced a much more potent inhibitory effect (three-order shifting) on endothelin-1-induced contraction in internal mammary arteries, and also potently inhibited the contraction in radial arteries. 3. The ET(B) agonist IRL-1620 caused a contraction in internal mammary arteries, but not in radial arteries, although the response of radial arteries to endothelin-1 was very strong. The contraction induced by IRL-1620 was weaker than that induced by endothelin-1; however, the maximal contraction to IRL-1620 was obtained at 3 nmol/l, which was lower than that with endothelin-1 (maximal contraction at 10 nmol/l). 4. In internal mammary arteries the contraction to endothelin-1 and IRL-1620 gradually changed to relaxation with high concentrations of endothelin-1 (from 30 nmol/l) and IRL-1620 (from 3 nmol/l), whereas it did not in radial arteries; suggesting that the ETB receptor on human arterial smooth muscle cells may mediate contraction at low agonist concentrations and relaxation at high agonist concentrations. 5. The ET(B) agonist IRL-1620, endothelin-1 and endothelin-3 did not cause endothelium-dependent relaxation in either precontracted radial arteries or internal mammary arteries, although endothelium-dependent relaxation was fully induced by acetylcholine in these two arterial preparations. 6. In conclusion, the present studies demonstrate that the responses of internal mammary arteries and radial arteries to an ET(B) antagonist and an ET(B) agonist are significantly different from those of animal vascular vessels, and also from each other. The ET(B) receptor may play only a minor role in endothelium-dependent relaxation of these human arteries. Endothelin-1-induced contraction is mediated by both the ET(A) (major) and the ET(B) (minor) receptors in internal mammary arteries, but only by the ET(A) receptor in radial arteries. These studies may help to determine therapeutic strategy.