The plasma concentration of atrial natriuretic peptide (pANP) was investigated at fixed times during 48 h following the onset of an initial attack of mild acute myocardial infarction (AMI) in 11 patients. Six hours after onset, the mean pANP was elevated, but 6 h later pANP had returned to the baseline level. Forty-eight hours after onset the mean pANP increased again. Thus, the curve of the time course of pANP consisted of 2 peaks separated by a dip. Six hours after onset, systemic hemodynamics and humoral factors were measured in 27 patients. At 48 h they were measured in 14 patients. The late elevation of pANP correlated with mean pulmonary arterial wedge pressure (r = 0.63, n = 14, p less than 0.05), while the early elevation of pANP did not (r = 0.31, n = 27, n.s.). The early elevation of pANP correlated with plasma concentrations of both noradrenaline (r = 0.55, n = 27, p less than 0.01) and creatine phosphokinase (r = 0.54, n = 27, p less than 0.01). In addition, noradrenaline levels positively correlated with mean pulmonary arterial wedge pressure (r = 0.38, n = 27, p less than 0.05). The cause of the early elevation of pANP in AMI is unclear, but it is suggested that injury to myocardium and activated sympathetic nerve activity may be responsible in part.