We evaluated the concentrations of the brain and atrial natriuretic peptides in the plasma as markers of ventricular function and volume load in children with functionally univentricular hearts. We studied 7 children aged from 0.5 to 0.7 years with functionally univentricular hearts who had undergone a first palliative operation, and 10 children aged from 1.8 to 3.7 years who had undergone a bidirectional Glenn anastomosis at ages ranging from 0.4 to 1.0 year. As a control group, we studied 14 children without heart defects aged from 0.1 to 4.5 years. Levels of the brain natriuretic peptide were measured at 8.3 to 122 ng/l, with a mean of 52.8 ng/l, after the first palliative operation, compared to 0 to 16 ng/l, with a mean of 7.3 ng/l, after a bidirectional Glenn anastomosis, and 0 to 13.8 ng/l, with a mean of 5.9 ng/l, in the children serving as controls. Corresponding values for atrial natriuretic peptide were 17 to 203 ng/l, with a mean of 103 ng/l, after the first palliative operation, compared to 16 to 54 ng/l, with a mean of 29 ng/l, after the bidirectional Glenn anastomosis, and 12 to 52 ng/l, with a mean of 32 ng/l in the controls. Echocardiography showed that all the children with functionally univentricular hearts had normal ventricular function. Blood presssure, pulmonary arterial pressure, and arterial saturations of oxygen did not differ between the groups. We conclude, that in children with functionally univentricular hearts, the volume overload imposed on the heart after the first palliative operation is associated with increased production of brain and atrial natriuretic peptides, while after ventricular unloading, levels of the natriuretic peptides return to control values.