The mechanisms underlying the increased ventilatory response to exercise seen in patients with chronic heart failure are not clearly understood. Arterial potassium has been suggested as an important ventilatory stimulant. The authors have investigated the arterial potassium response in patients with heart failure. Although arterial potassium rises during exercise, no evidence was found to suggest a greater potassium response in patients with heart failure compared to normal subjects. There was no direct correlation between the rise in ventilation and the rise in arterial potassium. It remains possible that there is an increased sensitivity to arterial potassium in patients with heart failure, but it would need to be three times greater than in normal subjects.