A disturbance in the chronotropic response to exercise has well documented following surgical repair of a congenital heart defect. Observations of a similar dysfunction following correction of simple atrial as well as ventricular septal defect suggest that its occurrence might be linked to a common denominator in the surgical procedures. Since cardiopulmonary bypass (CPB) is used for all initacardiac surgeries, the contribution of this factor to the abnormal response was examined. Three groups of patients von; evaluated: VSDop: operated on for a ventricular septal defect, CPB required; VSDnon-op: spontaneous closure of a congenital 73D, no surgery; and PDAoperated on for a patent ductus arteriosus, no CPB required. They were compared io a control group of healthy adolescents (C). The main, finding was that of lower submax final and maximal heart rates in VCDop disc in any other group. These results suggest that placement of cannulae resulting from-CPB could alter the control of heart rate during exercise and contribute to the abnormal chronotropic response following intracardiac repair of a congenital heart defect.