The haemodynamic effects of hydralazine were studied in seven infants and a child, each with a large ventricular septal defect. Hydralazine, 0.3 mg/kg, was administered intravenously. This caused a lowering of pulmonary arterial pressure from 57 +/- 4 mmHg to 49 +/- 4 mmHg, and a lowering of left atrial pressure from 12 +/- 1 mmHg to 10 +/- 1 mmHg. Systemic vascular resistance was reduced from 19.0 +/- 2.4 units/m2 to 15.1 +/- 0.9 units/m2. Pulmonary vascular resistance was reduced in six cases and increased in two. Systemic blood flow (Qs) increased in six cases and was unchanged in one. It decreased in one case where the pretreatment Qs was high, that is 6.4 l/min per m2. Pulmonary blood flow (Qp) increased in six cases while it decreased in two. These two cases had a pretreatment Qs less than 3.0 l/min per m2. The pulmonary to systemic blood flow ratio (Qp/Qs ratio) decreased only in three patients who had Qs less than 3.0 l/min per m2 before hydralazine. In contrast, the Qp/Qs ratio increased in the five cases with higher pretreatment Qs. Thus, this study has found that the pretreatment Qs alters the effect of hydralazine on the Qp/Qs ratio in large ventricular septal defects, thus indicating that afterload reduction treatment with hydralazine may be effective in the management of large ventricular septal defects by reducing the Qp/Qs ratio in the cases that are associated with a low systemic blood flow.