In a randomized double-blind study, treatment with either metoprolol, nifedipine, or their combination was compared for effects on ischemic variables and heart rate obtained during ambulatory monitoring in 42 patients with chronic stable angina. All patients had severe chronic stable angina of at least 6 months' duration despite medical treatment, and exhibited coronary artery stenosis of 75% in one or more coronary arteries. Metoprolol reduced the frequency of total (p less than 0.01) and asymptomatic ischemic episodes (p less than 0.05), the duration of ischemia (p less than 0.05), and the ischemic burden (p less than 0.05), which contrasted to the lack of any similar significant effect during nifedipine monotherapy. During combination therapy, there was a tendency to further improvement, which did not reach statistical significance compared with metoprolol monotherapy. Heart rate at the onset of ischemia was reduced by metoprolol therapy (p less than 0.01), indicating that metoprolol acts by reducing myocardial oxygen demand even during ischemic episodes observed in daily life, where impairments of myocardial oxygen supply are suspected. No change in heart rate at the onset of ischemia could be detected during nifedipine monotherapy. It is concluded that metoprolol monotherapy, as well as its combination with nifedipine, effectively reduces total ischemic activity compared with placebo and nifedipine monotherapy. Control of ischemic activity in chronic stable angina may have prognostic implications.