In children with congenital cardiac anomalies, isoproterenol has been used to stimulate exercise. To determine if these states are comparable, 28 children with pulmonary valvar stenosis were studied by cardiac catheterization at rest, during supine submaximal exercise, and during isoproterenol infusion. Similar heart rates were obtained during exercise (142/min) and isoproterenol infusion (139/min). Cardiac index, right ventricular systolic pressure and oxygen consumption were measured during each physiologic state. The change in pulmonary valve area from rest was calculated. At similar heart rates, cardiac index was significantly greater with exercise than with isoproterenol and right ventricular systolic pressure was similar in the two states. Thus, the calculated pulmonary valve area was significantly smaller with the pharmacologically induced stress. The study indicates that several of the hemodynamic effects of isoproterenol are not comparable to those produced by exercise in patients with pulmonary valvar stenosis.