The effect of a standardized psychological stress test on frequency and severity of ventricular extrasystoles was compared in 42 patients with the results of 24-hour ECG monitoring and of an exercise ECG test. Frequent ventricular extrasystoles (greater than 1/min) occurred in the 24-hour-ECG in 22, in the psychological stress test in 24 and on physical exercise in 22 patients. 24-hour monitoring gave the only positive finding in ten, the psychological stress test in six, the exercise test in four. Complex arrhythmias (coupled or runs of ventricular extrasystoles) occurred in the 24-hour ECG in 30, with the psychological stress test in 16 and the exercise test in 12 patients. The 24-hour ECG was alone positive in 16, the psychological test in two, the exercise test in one. Thus in an individual patient the psychological stress test may provoke additional arrhythmias which are not recorded in either the 24-hour ECG or after exercise. The findings point to an independent significance of the psychological stress test in the diagnosis of simple and complex ventricular arrhythmias.