Plasma renin activity (PRA) was measured in 23 patients (pts.) affected by myocardial infarction (AMI), and in a control group of 4 normal subjects, during bed rest. A blood sample was drawn every 12 hours to determine PRA pattern curve during 5 days in Coronary Care Unit. The initial PRA was in the normal range in all pts., but it rose subsequently. The highest level was reached on the 4th day. The mean value of PRA at this time was significantly higher than the initial one (p less than 0.01). Such increasing PRA was not correlated with age, sex, infarction site, infarction dimension, pain, blood pressure, arrhythmias, bed rest. At variance with the experience of others, we were not able to show a significant correlation between PRA and plasma K+ levels. Diuretic and/or beta-blocker agents too, did not influence this increase of PRA in AMI. Our results are compared with those previously reported in the literature and the possible pathophysiological mechanisms are discussed.