To study the direct effects of nocturnal hypoxemia on the heart, we recorded electrocardiographic tracings and arterial oxygen saturation (SaO2) during the night in patients with chronic obstructive airways disease. In seven "blue-bloater" patients the mean basal SaO2 was less than 80 per cent, and it fell by more than 10 per cent during 29 episodes of transient hypoxemia. Only five such episodes occurred in three of five "pink-puffer" patients. All "blue bloaters" with low basal mean SaO2 had multiple atrial and ventricular premature contractions and a high heart rate at rest; six patients has a prolonged QTc, three had ST-T depression, and one had right-bundle-branch block. Oxygen therapy increased basal mean SaO2, reduced ectopic activity, abolished ST-T changes and bundle-branch block, significantly reduced the resting heart rate and the amplitude of the R and S waves, and shortened the QTc in four nonsmokers. These results suggest that sustained hypoxemia contributes to myocardial dysfunction and heart failure in "blue-bloater" patients.