Adenosine, a purine nucleoside, plays a variety of roles in cardiovascular and ventilatory control, and may be a marker of tissue hypoxia. There is, however, no direct evidence of an increase in plasma or in tissue levels of adenosine during moderate hypoxia in humans. We measured the plasma concentrations of adenosine in an artery and the median cubital vein simultaneously in 12 normal volunteers, and also in the internal jugular vein in seven of them during normoxia and moderate hypoxia (SaO2 = 80%, 20 min) with or without dipyridamole (0.6 mg/kg) pretreatment. Dipyridamole was expected to block reuptake of adenosine by red blood cells and vascular endothelial cells so that the plasma level of adenosine would more likely reflect the tissue level. Blood was sampled with an appropriate stopping solution, and adenosine was measured with a high-pressure liquid chromatographic (HPLC)-fluorometric technique. The plasma concentration of adenosine did not rise either in the artery or in the vein at any phase of hypoxia without the dipyridamole pretreatment. However, when subjects were pretreated with dipyridamole, the plasma concentration of adenosine increased significantly and markedly in a time-dependent manner during hypoxia in the vein, but not in the artery. The adenosine level rose from 20. 7 +/- 2.5 nM (mean +/- SE) during normoxia to 50.7 +/- 10.7 nM at 20 min of hypoxia, and returned to the baseline level in the recovery phase. The plasma concentration of adenosine in the jugular vein did not change during hypoxia either with or without dipyridamole pretreatment. These data provide evidence that in humans, the local production of adenosine increases during moderate hypoxia in forearm tissue, although this is not reflected in plasma unless the subject is pretreated with dipyridamole.