In the present study, responses of long-term human coronary artery bypass grafts (CABGs) to known endothelium-dependent vasodilators, acetylcholine, calcium ionophore A23187, thrombin, and histamine, as well as authentic nitric oxide, the putative endothelium-derived relaxing factor, were studied. Sixteen CAGBs were isolated within 1-2 hours from hearts of 14 patients receiving a cardiac transplant. A total of 109 ring segments were prepared from these CABGs and studied in vitro. The duration of the CABGs ranged from 7 months to 12 years. Addition of acetylcholine (0.01-10 μM), calcium ionophore A23187 (0.01-1.0 μM), thrombin (0.01-1.0 unit/ml), and histamine (0.01-1.0 μM) consistently produced a dose- and endothelium-dependent relaxation, reaching a maximum of -35.3 ± 3.3%, - 45.3 ± 5.5%, - 26.9 ± 4.8%, and -17.8 ± 2.5% (mean ± SEM), respectively. No significant difference was observed among the CABGs with different duration of transplantation, whereas the relaxant responses of different segments along the entire length of a CABG were markedly different. These latter differences in the endothelium-dependent responses appear to correlate inversely with the development of intimal proliferative lesions in these CABGs. Addition of nitric oxide (0.01-10 μM) produced a potent dose- and endothelium-independent relaxation, which was also slightly depressed in CABGs with severe intimal proliferation. These results demonstrate that long-term transplanted human saphenous vein grafts retain their endothelium-dependent responses and that development of severe intimal proliferative lesions, rather than the duration of the grafts, result in marked alterations in the reactivity of these transplanted CABGs.