Mortality rises as resting heart rate increases. Patients who are less able to increase their heart rate while exercising are more likely to die than those whose heart rate does rise. The occurrence of silent ischaemic events shows a circadian distribution, with increased mortality, heart rate and systolic blood pressure early in the morning. Treatment with beta-blockers reduces the number of asymptomatic ischaemic episodes, and lowers heart rate during exercise and at rest. The ischaemic myocardium of the dog loses the ability to undergo wall thickening during exercise. Moreover, blood flow in the ischaemic subendocardium is reduced during exercise. These effects can be overcome using beta-blockers. The effects of beta-blockers in reducing heart rate are less pronounced during mental stress. Furthermore, changes in wall motion abnormalities during mental stress are not improved by beta-blockers. Treatment of patients with beta-blockers after a myocardial infarction reduces the mortality rate. The greater the reduction in heart rate, the greater the reduction in mortality.