Large fluctuations in systemic arterial potassium have been found during and after exercise in normal subjects. To determine whether similar changes occur in patients with angina pectoris, arterial potassium levels were measured before, during and immediately after maximal bicycle exercise in 20 patients with exertional angina. In 10 of these patients, leg blood flow and arteriovenous potassium levels also were measured. During exercise, arterial potassium increased significantly both from rest to submaximal exercise (4.3 +/- 0.1 to 4.7 +/- 0.1 mmol/liter, p less than 0.01) and from submaximal to maximal exercise (5.4 +/- 0.1 mmol/liter, p less than 0.01). Within 1 minute of cessation of exercise, arterial potassium had decreased to 4.7 +/- 0.1 mmol/liter (p less than 0.001) and continued to decrease to a minimum of 4.1 +/- 0.1 mmol/liter between 3 and 5 minutes after exercise, significantly less than the rest value (p less than 0.05). At maximal exercise (99 +/- 9 watts), the calculated release of potassium from each leg reached 2.7 +/- 1.3 mmol/min. Four minutes after exercise, the leg muscles were resorbing potassium at 0.24 mmol/min. In these patients with exertional myocardial ischemia, the magnitude and rapidity of arterial potassium changes during and after exercise resemble those found in normal subjects, but occurred at much lower workloads. Release and resorption of potassium by exercising muscle in patients with angina pectoris may cause potentially arrhythmogenic arterial potassium fluctuations.