Cardiovascular dynamics at the onset of exercise. Med. Sci. Sports Exerc., Vol. 31, No. 7, pp. 1005-1010, 1999. At the onset of exercise, the cardiovascular system adapts with a series of integrated responses to meet the metabolic demands of the exercising muscles. The importance of rapid increases in cardiac output and local muscle blood flow has been established by showing that small decreases in O2 supply at the onset of exercise cause delays in the increase in O2 utilization. With the development of techniques that can be applied to instantaneous measurement of the cardiovascular response, the mechanisms that regulate increased muscle perfusion have recently been investigated. In this introduction to the symposium, a model of the within muscle distribution of blood flow is considered as a function of the measured responses across an exercising skeletal muscle. This model demonstrates the necessity of considering the distribution of blood flow to the working fibers very early in exercise. The symposium provides insight into our current understanding of the factors involved in the regulation of skeletal muscle blood flow at the onset of exercise. Our ability to study the impact of cardiovascular disease on exercise performance has improved with the development of selective pharmacological agents to block or stimulate specific components of the cardiovascular response. These advances should provide the basis for future research.