On the basis of previous data suggesting the involvement of cardiac histamine in ischemic heart disease (IHD), we evaluated plasma histamine (H) and creatine-kinase isoenzyme (CK-MB) level in cardiac and healthy subjects. 20 patients with acute myocardial infarction (AMI) (10 developing AMI in Hospital, thus making possible the detection of plasma H level before acute event), 10 patients with IHD not developing AMI and 10 presumably healthy subjects were admitted to the study. 15 of all patients with AMI showed a correlated H and CK-MB trend during AMI reaching the highest peak 24 hours after onset of pain. 7 of the patients with IHD who developed AMI in Hospital showed a slightly higher plasma H level, before AMI, than those with IHD who did not develop AMI. A possible role of histamine in the pathogenesis of AMI is discussed.