Concentration of atrial natriuretic peptide (ANP) was studied in acute myocardial infarction (AMI), its association with ventricular arrhythmias (VA), left ventricular dysfunction and infarct size. Plasma ANP concentrations were measured at time: 0, 4, 8, 16, 24, 48 and 72 hours after admission in 11 patients (pts) with first AMI, up to 6 hours after the first symptoms. Ventricular arrhythmia was assessed by 24 hour Holter monitoring, left ventricular dysfunction by echocardiography and infarct size by serial CK-MB measurement in first 72 hours of AMI. In subsequent measurements the average plasma concentration of ANP (mean +/- SEM) was elevated: 42.2 +/- 9.9, 35.3 +/- 12.5, 33.9 +/- 8.3, 42.3 +/- 8.3, 36.9 +/- 6.4, 60.7 +/- 9.3, 47.8 +/- 12.0 fmol/ml. The maximal plasma ANP concentration was significantly higher (p less than 0.01) in 4 pts with VA 4th grade acc. to Lown than in 7 pts without VA (102.6 +/- 17.9 vs 41.1 +/- 6.4). The maximal level of ANP--153.3 fmol/ml in a patient with paroxysmal supraventricular tachycardia was observed. There were no significant correlations between plasma ANP and infarct size, size of left atrium and contractility disturbances of left ventricle.