In an attempt to elucidate the pathophysiological significance of the sympathetic hyperactivity in the acute stage ofmyocardial infarction, the author observed changes in the urinary excretion of CA, the CA content in the myocardium and the hemodynamics in both clinical and experimental myocardial infarction, and the following were found: 1) In clinical myocardial infarction, the urinary excretion of CA was markedly increased immediately after an attack, and the assay ofmyocardial specimens form the autopsied patients of acute myocardial infarction revealed that the CA content in the non-infarcted area was lower than that in the infarcted area. 2) In the experiments on rabbits with ligated coronary artery, the increase in cardiac contractility and rise in blood pressure in response to CA was suppressed after the ligation of coronary artery. In the early stage of experimental myocardial infarction, the decrease of myocardial CA content in the non-infarcted area was as in autopsied patients, predominant over the decrease of that in the infarcted areaL In the chronic stage (more than one week after the coronary ligation), the CA content in the infarcted area showed further decrease, but in the non-infarcted area it was recovered to the level in the control animals. The uptake of exogenous NA into the non-infarcted area decreased in the acute stage, and in the infarcted area it showed marked decreased in the chronic stage. The urinary excretion of CA was increased in the acute stage of myocardial infarction. 3) The administration of betamethasone suppressed the decrease in the CA content in the myocardium following the ligation of coronary artery. Based on these findings, the author came to a postulation that the sympathetic hyperactivity which is suggested by increased urinary excretion of CA and decreased CA content in the myocardium results from the reasonable biophylactic reaction so as to supplement the cardiac hypofunction derived from myocardial infarction.