We have studied the effect of beta-adrenoceptor blockade on the pyrexial response to acute myocardial infarction (AMI). Temperatures of 22 patients treated with timolol, 20 mg daily, in the acute phase of AMI were compared to temperatures of 22 patients not receiving timolol treatment. Fever response after AMI was significantly reduced in the timolol-treated patients, maximal and mean temperature in the febrile period being lower and the febrile period being shorter. Reduction of fever in AMI may be of importance as a higher body temperature increases the risk of developing heart failure and arrhythmias. The study lends support to the use of beta-adrenoceptor blockade in AMI. It also substantiates a possible role of beta-adrenergic receptors in the regulation of body temperature during fever.