Based on data from a variety of experiments from several laboratories, adenosine appears to play an important role in the adjustment of blood flow to the metabolic requirements of the tissue. This has been shown to be true for heart, brain, and skeletal muscle in several different species. A reduction in oxygen supply or an increase in oxygen demand results in vasodilation and adenosine release. However, adenosine is also coupled to blood flow increments with enhanced metabolic activity and in the presence of an adequate oxygen supply. To what extent other vasoactive agents participate with adenosine in producing vasodilation under a variety of conditions is not known.