Twenty patients with congenital heart disease, ages 8 months to 6 years, received 1 to 5 micrograms/kg/min iv nitroglycerin after intracardiac repair to assess the hemodynamic effects of, and the pulmonary vascular response to, the drug in pediatric cardiac patients. Before surgery all patients had systemic-to-pulmonary blood flow ratios equal to or greater than 2.5:1. The pulmonary arterial pressure (PAP) was equal to aortic pressure in 14 patients and normal in six. The pulmonary vascular resistance (PVR) was elevated (more than 4 units) in five patients. After surgery, the right and left atrial pressures, PAP, and cardiac index were measured by thermodilution, and the systemic vascular resistance (SVR) and PVR were calculated. Intravenous nitroglycerin produced a significant drop in SVR at higher doses only (1602 +/- 359 to 1590 +/- 324 dynes X sec X cm-5, p = NS, at 2 micrograms/kg/min vs 1602 +/- 359 to 1247 +/- 417 dynes X sec X cm-5, p less than 0.01, at 5 micrograms/kg/min). Similarly, improvement in cardiac index was apparent only at higher doses (3.33 +/- 1.25 to 3.49 +/- 0.90 liters/min/m2, p = NS, at 2 micrograms/kg/min vs 3.33 +/- 1.25 to 3.82 +/- 0.98 liters/min/m2, p = NS, at 5 micrograms/kg/min). Also, the effect on the pulmonary vascular bed was most marked at higher doses and in patients with elevated PAP (175 +/- 27 to 148 +/- 28 dynes X sec X cm-5, p = NS, in patients with normal PAP vs 305 +/- 78 to 214 +/- 76 dynes X sec X cm-5, p less than .001, in patients with elevated PAP).(ABSTRACT TRUNCATED AT 250 WORDS)