To determine if maximal exercise treadmill testing influences the occurence of ventricular arrhythmia in the hours after exercise, 45 myocardial infarction and 22 angina pectoris patients (New York Heart Association Class I-II), and 23 normal subjects were examined with 24-hour ambulatory electrocardiographic Holter recordings before and after exercise testing. Comparison of qualitative and quantitative ventricular arrhythmia detected during identical chronological two-, four-, and 20- or more hour periods, before and after exercise testing in each patient, revealed no statistically significant difference in any patient group. The prevalence of ventricular ectopy in 80 per cent of ischemic heart disease patients and 30 per cent of normal subjects as detected by 24-hour Holter recordings was similar to previous studies. It is concluded that in ambulatory ischemic heart disease patients (New York Heart Association Class I-II) and normal subjects, maximal treadmill testing does not significantly affect the occurrence of ventricular arrhythmia in the hours after exercise.