One hundred forty-four patients admitted to the hospital within four hours after onset of symptoms of myocardial infarction were randomly assigned to either intravenous timolol treatment or to placebo. Timolol was given intravenously for the first 24 hours and orally thereafter for the duration of hospitalization. Infarct evolution was assessed by continuous vectorcardiography and creatine kinase release. The timolol group had reduced myocardial ischemia and infarct size as measured by an accelerated reduction of ST-vector magnitude, a significant reduction of maximal cumulative creatine kinase release (29.5 per cent), and significantly smaller changes in QRS-vector variables (20 to 25 per cent). Furthermore, the predicted creatine kinase release and maximal QRS-vector change for a given initial ST-vector magnitude was significantly reduced in the timolol group. Timolol was also associated with significant reductions in pain and need for analgesics and was well tolerated overall. This study supports the use of intravenous timolol in the early phase of suspected myocardial infarction to limit infarct size.