The extensibility of the pulmonary artery (PA) were examined using a combined hemodynamic and ultrasonic technique in 14 patients with atrial septal defect (ASD), three with ASD and pulmonary hypertension (PH), 17 with ventricular septal defect (VSD) and 11 with VSD and PH. Seven patients with neither intracardiac shunt nor PH served as controls. The tension (T) of PA was measured as the product of the pressure and the diameter (D) and was found to be high in the patients with intracardiac shunt. The resting diameter of PA (D0) was calculated as the intersect with the D axis in the regression line of PA tension-diameter relation. The Lagrangian strain of PA [(D--D0)/D0] was obtained using D0 and correlated with T yielding a linear regression equation. The elastic modulus (E) was obtained as the slope of this equation. E in the patients with ASD [(1.0 +/- 0.5) x 10(5) dynes/cm] or VSD [(9.8 +/- 4.6) x 10(4) dynes/cm] were slightly higher than that of the control [(6.7 +/- 2.9) x 10(4) dynes/cm], but these differences were not statistically significant. However, E in the patients with ASD and PH [(2.5 +/- 1.3) x 10(5) dynes/cm] or in the patients with VSD and PH [(2.6 +/- 0.9) x 10(5) dynes/cm] were significantly higher than the control. E seemed unlikely to be causally related to the resistance of PA capillary beds, and was suggested to be influenced by the histological changes of the pulmonary trunk. It was concluded that the extensibility of PA in the patients with ASD or VSD was not altered secondary to the increased blood flow unless PH was associated.