Contractile and electrical responses of muscle strips from rabbit aorta to 5-HT and hyper K solution were compared in controls group of rabbits fed a normal diet, and atherosclerotic animals fed 1% cholesterol diet for 12 weeks (atherosclerotic group). A study of vascular responses of human popliteal artery was also carried out.It was shown that smooth muscle cells from arteriosclerotic vessels of both rabbits and human exhibit an increased sensitivity to contractile action of hyperpotassium solution and 5-HT. We found no significant difference in the densities of L-type Ca²⁺ channel current as well as voltage dependent K+ channels current measured by patch clamp method in single smooth muscle cells from control and atherosclerotic rabbit aorta. Removal of endotelium from the vascular strips resulted in an enhanced vascular contractility in control but not in atherosclerotic group. The dose response curves for endothelium-free muscle strips for both 5-HT and hyper K solution were not significantly different between the two groups. Our result suggest that the enhanced vascular responses to 5-HT and hyper K under condition of arteriosclerosis could be attributed to an impaired function of endothelium which is a source of substances particularly nitric oxide that mediates vascular relaxation and provides a compensatory mechanism to keep periferal resistance at lower level.