Endothelial cells from coronary vessels of guinea pig hearts were isolated, cultivated and morphologically characterized. --Cells from confluent cultures contained adenine nucleotides and their dephosphorylated degradatives in exceptionally high amounts. --Adenine nucleotide levels were only slightly influenced by the growth state of the cultures and remained stable during incubation for three days in purine-free medium. In contrast, brief incubation of endothelial cells under anoxic conditions resulted in a substantial breakdown of adenine nucleotides associated with an enhanced formation and release of adenosine. --Measurements of specific activities of enzymes involved in adenine nucleotide synthesis and degradation lend additional support to the view that a very active adenine nucleotide metabolism is a typical feature of cultured coronary endothelial cells.