A 70 percent reduction in the rate of blood flow through the common carotid artery in rabbits caused a 21 percent decrease in the diameter of this artery within 2 weeks. The smooth muscle relaxant papaverine did not attenuate the response; therefore, such reductions in diameter probably reflect a structural modification of the arterial wall rather than sustained contraction of smooth muscle. This arterial response to reduced blood flow was abolished when the endothelium was removed from the vessels. It appears that the endothelium is essential for the compensatory arterial response to long-term changes in luminal blood flow rates.