Right atrial pacing at different rates was performed on 15 women and 11 men with persistent angina with normal resting and exercise ECG. In 21 subjects there developed during bicycle exercise unequivocal S-T depression of myocardial ischaemia at maximal frequency. In 16 of them the diagnosis was confirmed by rate-dependent attacks of angina. These results demonstrate that, especially in the elderly, the maximal rate (+/-15%) thought to give a full load to cardiovascular functions is not suitable for diagnosing with certainty coronary insufficiency on exercise. In such patients rate-stimulation test may help in recognising the basic disease. On the other hand, in younger patients, capable of marked increase in heart rate on maximal exercise, rapid atrial stimulation has no diagnostic advantage. This test is indicated in those subjects who during ergometric exercise have an insufficient increase in heart rate because of a general reduction in exercise tolerance.