Neurohumoral parameters in 45 asymptomatic patients with acute myocardial infarction were measured. In patients with mild left ventricular dysfunction (ejection fraction < or = 45% and/or left ventricular end-diastolic pressure > or = 15 mm Hg), atrial natriuretic peptide levels differed significantly from those in patients with normal left ventricular function (127 +/- 74 pg/ml vs. 79 +/- 71 pg/ml; p < 0.001). After stimulus an increase in atrial natriuretic peptide levels (79 +/- 71 to 118 +/- 86 pg/ml; p < 0.001) was found only in patients with normal hemodynamic parameters. These data show that in patients with left ventricular dysfunction, the atrial natriuretic system is activated; however, atrial natriuretic peptide response to stimulus is present only in patients with normal hemodynamics.