Recently, there have been many reports on the role of serum concentration of potassium as a potential limiting factor during exercise. K+ is known to induce muscle fatigue and to increase ventilation by direct stimulation of peripheral chemoreceptors. These two K(+)-mediated effects are considered to be the factors that limit exercise capacity. This effect seems to be exacerbated in hypoxemic states. The occurrence of hypoxemia in chronic pulmonary disease patients during exercise is believed to cause the excessive efflux of potassium to the extracellular space from skeletal muscles. The present study evaluated the relationship between the changes in arterial concentration of K+ and hypoxemia during exercise in 37 chronic pulmonary patients and 9 healthy controls. The study was carried out after obtaining formal and written consent of the patients in accordance with the stipulations of the Helsinki Declaration. Patients were divided into two groups according to PaO2 level at maximum exercise capacity; group I: PaO2 less than 55 torr, group II: PaO2 greater than or equal to 55 torr. The arterial concentration of K+ in group I patients tended to be higher than that in group II and controls. In group II, K+ concentration was significantly elevated (p less than 0.05) compared to controls. In group II, PaO2 values were inversely correlated with K+ (r = 0.3026; p less than 0.025), whereas in controls and group II they were unrelated. These results suggest that the augmented serum level of potassium in patients with chronic pulmonary disease is an important limiting factor during exercise.