Cultured coronary endothelial cells and the coronary endothelium of isolated perfused guinea-pig hearts are characterized by a very active adenosine and adenine nucleotide metabolism. Adenosine applied to the endothelium at low concentrations is avidly metabolized and preferentially incorporated into different nucleotide pools--only a minor amount is degraded to uric acid. Physiologically, the coronary endothelium therefore functions as an impermeable metabolic barrier for interstitially or intravascularly accumulating adenosine. Only at concentrations greater than or equal to 10(-6) M adenosine can pass the endothelial barrier. As a consequence, the vasodilatory action of adenosine formed in or administered into the coronary system cannot be induced by a direct association of the nucleoside with the putative adenosine receptor of the arteriolar smooth muscle cells, but must be mediated by the endothelium. High molecular weight derivatives of adenosine, clearly confined to the coronary system, can also induce a coronary dilation. The endothelium-mediated smooth muscle relaxation is therefore obviously due to triggering of an extracellular adenosine receptor at the luminal surface of the endothelium. Since this process is accompanied by a rapid and pronounced activation of the adenylate cyclase system, the endothelial receptor conforms to an A2-type. According to our results it is necessary to reconsider qualitative and quantitative facets of the adenosine hypothesis of metabolic regulation of coronary blood flow, which--in its original formulation--exclusively centers on the cardiomyocyte metabolism. With respect to the vasoactivity of adenosine one obviously has to distinguish between its action from the interstitial space directly via the myocyte receptors of the vessel wall, and/or its action from the intracoronary space via the newly detected endothelial A2-receptor. More information is needed to determine the extent to which both receptor populations actually participate in the metabolic regulation of coronary flow under physiological and pathophysiological conditions.