The purpose of this study was to measure changes in serum atrial natriuretic factor concentrations immediately after heart operations in children under baseline conditions and in response to continuous infusion of dopamine (2.5 and 5.0 micrograms/kg/min). During control periods, levels of atrial natriuretic factor were elevated at 190 +/- 24 and 199 +/- 36 pg/ml. The cardiac index was 2.6 L/min/m2 and the renal plasma flow was decreased to 269 +/- 41 ml/min/1.73 m2, indicating a state of renal vasoconstriction (mean renal fraction of cardiac index of 10.0% +/- 1.0%). The mean sodium fractional reabsorption was 99.0% +/- 0.2%. During dopamine infusion, atrial natriuretic factor concentrations increased to 259 +/- 57 pg/ml and to 280 +/- 56 pg/ml, with dopamine 2.5 and 5.0 micrograms/kg/min, respectively (p = not significant), whereas left atrial pressure decreased from 11.7 +/- 0.9 mm Hg during the control period to 10.1 +/- 0.9 and to 9.9 +/- 1.0 mm Hg (p less than 0.05). No correlation was found between changes in left atrial pressure and atrial natriuretic factor levels. Dopamine at 5 micrograms/kg/min increased the cardiac index to 3.0 +/- 0.2 L/min/m2 (p less than 0.001) and the renal plasma flow to 406 +/- 61 ml/min 1.73 m2 (p less than 0.001), alleviating the renal vasoconstriction. The mean urinary sodium excretion increased to 0.33 +/- 0.08 mmol/kg/hr (p less than 0.01). The atrial natriuretic factor plasma concentrations were not related to the urinary sodium excretion, renal plasma flow, or glomerular filtration rate during the control period or during dopamine treatment. These data indicate that after heart operations in children, low urinary sodium excretion occurs despite high circulating atrial natriuretic factor levels. Atrial natriuretic factor concentrations were related neither to left atrial pressures nor to the renal changes induced by dopamine.