ABSTRACT: This review summarizes and examines the evidence from experiments using animal models to determine the effect of endurance exercise training on endothelium-dependent dilation in the arterial circulation. The response of the endothelium to exercise training is complex and depends on a number of factors that include the duration of the training program, the size of the artery/arteriole, the anatomical location of the artery/arteriole, and the health of the individual. In healthy animals, short-term exercise training appears to cause enhanced endothelium-dependent dilation in some vascular beds, but it returns to normal levels as the duration of the training program increases. In general, evidence supports the notion that exercise training causes greater increases in endothelium-dependent dilation in various disease states than in healthy individuals. The evidence of a generalized effect of training on arterial endothelium in all regions of the body is inconsistent and appears to depend on the animal model used. Available results indicate that training duration, artery size, and anatomical location interact in ways not fully understood at this time to determine whether and to what extent endothelium-dependent dilation will be enhanced by exercise training.