Hypoxia has been reported to increase arterial potassium concentration ([K+]a) in anaesthetized cats (Paterson, Estavillo & Nye, 1988). The purpose of this study was to determine whether this phenomenon occurs in humans. The effect of hypoxia on [K+]a was measured in ten male subjects, at rest and during light exercise, before and after 8 weeks of physical training. The [K+]a increased by 0.15 +/- 0.04 mM (mean +/- S.E.M.) at rest, when end-tidal PO2 (PET,O2) was lowered from 100 to 51 +/- 1.6 Torr, and by 0.10 +/- 0.02 mM during exercise, when PET,O2 was lowered from 100 to 66 +/- 6.2 Torr. Physical training did not alter the rise in [K+]a significantly. The magnitude of this effect is small in comparison with that of exercise-induced hyperkalaemia, and is unlikely to be of great physiological significance at moderate levels of hypoxia.