The response to intravenous administration of propranolol hydrochloride was studied in 24 children undergoing operation for isolated or complex right ventricular outflow tract obstruction. Ten had pulmonary valve stenosis, 4 had pulmonary valve stenosis plus ventricular septal defect, 8 had tetralogy of Fallot, and 2 had complex lesions. Propranolol (0.01 to 0.2 mg per kilogram of body weight) was given to patients with a residual right ventricular to left ventricular systolic pressure ratio greater than 0.75. Four children did not respond to propranolol and required further surgical intervention. Twenty patients responded, 2 of whom died. Fifteen were restudied by cardiac catheterization 3 weeks to 27 months postoperatively. The right ventricle-pulmonary artery gradient was 25 mm Hg or less in 13 patients. We conclude that a small dose of propranolol given intraoperatively can identify patients in whom a reduction in the right ventricle-pulmonary artery gradient to acceptable levels will occur in the months following operation. This may reduce the need for placement of a subannular or transannular patch in some patients.