A 21-yr-old man with multiple pulmonary AV fistulae presented with moderate hypoxemia with chronic hyperventilation, which continued even after sufficient oxygenation by 100% O2 inhalation. The infusion of aminophylline, an adenosine receptor blocker, not only increased PaCO2 from 35 to 39 mm Hg but also depressed the magnitude of hypoxic ventilatory response (HVR). After a surgical removal of the major fistulae, PaCO2 increased to 42 mm Hg. At this time, aminophylline increased the resting ventilation and slightly augmented the HVR value, which was opposite to the findings of preoperation. Dipyridamole, which potentiates endogenous adenosine by inhibiting its cellular uptake, increased the magnitude of HVR both before and after the operation. We propose that the hyperventilation observed in this case can at least in part be attributed to the ventilatory stimulation by endogenous adenosine, which presumably passed through the fistulae without being metabolized in the pulmonary circulation.