The authors compared by means of bicycle ergometry the effect of a single dose of 90 mg diltiazem (Dilzem Gödecke Co.) and placebo in 31 patients with chronic stable angina pectoris grade II-III according to NYHA. These patients were divided into group A where the pathomorphology of the coronary circulation made the "steal phenomenon" possible and group B where there were no conditions for the "steal phenomenon". Diltiazem, as compared with placebo, increased significantly in both groups the total work output during ergometric examination, delayed significantly the onset of stenocardia and the time before the development of a 1 mm depression of the ST segment; in group A it significantly reduced the depression of the ST segment during the maximum equal load achieved with diltiazem and placebo; it significantly reduced the diastolic blood pressure at rest and during exercise, it significantly reduced the heart rate in group B during exercise. The differences in the action of diltiazem, as compared with placebo, between the two compared groups A and B were not significant in any of the investigated parameters. Administration of diltiazem had a very favourable anti-anginal and anti-ischaemic action in both groups of patients, regardless of the morphology of the coronary affection.