Adenosine is a potent autocoid that acts as a vasodilator and modulator of inflammatory responses. Endothelial cells possess several mechanisms for altering circulating levels of adenosine and are capable of release of adenosine metabolites. We used cultured bovine aortic and main pulmonary arterial endothelial cells to determine whether endotoxin can alter adenosine uptake or release of adenosine metabolites. We found that 24 hours, but not 6 hours, of incubation with endotoxin caused endothelial cell injury, as assessed by cell detachment and chromium 51 release. Despite this injury the extent of [3H]adenosine uptake was unchanged. Using thin-layer chromatography to identify adenosine and its metabolites, we found that [3H]adenosine was primarily metabolized into intracellular hypoxanthine and adenine nucleotides. After 1, 6, and 24 hours of incubation with endotoxin there was an increase in extracellular adenosine metabolites, which was accompanied by decreases in the level of intracellular adenosine 5'-triphosphate. The appearance of adenosine metabolites in culture supernatants was a more sensitive measure of endothelial cell injury than 51Cr release or adherent cell number. The extracellular purine metabolite observed in response to endotoxin injury was mainly hypoxanthine. Our findings suggest that hypoxanthine release is an early event in endotoxin-induced endothelial cell injury. Because hypoxanthine may act as a substrate for xanthine oxidase, resulting in toxic oxidant production, its release has the potential of exacerbating vascular injury caused by endotoxin.