Changes in ECG during and after exercice were analyzed in 17 patients with exercise-induced asthma (ElA) and in 12 control patients (asthmatic patients without exercise-induced bronchial obstruction). The changes in ECG were compared with those in peak expiratory flow (PEF) rate and in arterial Pco2, Po2·and pH. It was found that in El A the amplitude of the P wave increased in the inferior leads and decreased in the aVL lead during and after exercise. In addition, the amplitude of the R wave diminished and the amplitude of the S wave increased in the anterior precordial leads during bronchoconstriction. The changes in the ECG of the control patients were small and were already beginning to return to normal 4 min after exercise, whereas the changes in El A patients persisted for 4–10 min after exercise. In El A, a significant negative correlation was found between the PEF rate and the amplitude of the P wave in leads II, III and aVF. In addition, the PEF rate and the amplitude of the S wave in the V3 lead showed significant negative correlation. Almost no changes were observed in Po2, or Pco2, in the EIA or in the control patients. The pH decreased significantly in both groups during exercise. The PEF rate did not correlate with arterial Po2, Pco2 or pH after exercise in E