Several studies have demonstrated a circadian rhythm in the onset of myocardial infarction. Most show a peak rate between 8 and 12 o'clock in the morning. The frequency of infarction in this time period is one and a half to two times that observed during the rest of the day. This phenomenon has been observed in different countries and does not seem to be influenced by the characteristics of the population (sex, previous infarction). However, this circadian rhythm seems less pronounced in younger subjects. The prescription of calcium antagonists before infarction does not change the circadian rhythm whereas aspirin and betablocker therapy decrease the frequency of matinal infarction. The mechanism underlying the circadian rhythm is hypothetical: many factors could play a triggering role and circadian variations in coagulation, platelet aggregation, catecholamine levels and blood pressure may also be important. Recent studies have shown that the morning peak in onset of infarction could be related to the times of waking and getting up. Suppression of the morning peak of infarction (and also of episodes of myocardial ischaemia and sudden death) with betablocker therapy, suggests a direct or indirect role of circulating catecholamines in the induction of the phenomena leading to infarction. When using betablockers, the authors suggest prescribing the doses so as to obtain therapeutic efficacy in the morning.