BACKGROUND: It has been known that a reversal of usual relationship between aortic and radial artery pressures(RAP) can occur in adult patients following cardiopulmonary bypass(CPB). The phenomenon of a pressure gradient between RAP and femoral artery pressure(FAP) were evaluated in pediatric patients before and after CPB. METHODS: 141 perdiatric patients undergoing open heart surgery were allocated into 2 groups. Group 1(n=77): infant's body weight was below 10kg. Group 2(n=64): child's body weight was between 10 and 20kg. After induction of anesthesia RAP was measured through 22G(1 inch) or 24G(3/4 inch) catheters and FAP was measured through 20G(2 inch) or 22G(1 inch) catheters using calibrated transducers. Hematocrit, rectal and nasopharyngeal temperature and left atrial pressure(LAP) were recorded 10 min after induction, immediately, l5, 30 and 60 min after CPB. Values are expressed as mean±SD and analysed using paired and unpaired t-test; p<0.05 was considered significant. RESULT: Systolic femoral arterial pressure(SFAP) was higher than radial arterial pressure(SRAP) before CPB in both groups. After CPB, the pressure gradient persisted in group 2 but was reversed with statistical significance in group l. CONCLUSION: When hypotension occurs during cardiac surgery, a comparison is recommended between radial and femoral or aortic pressure before treatment for hypotension is contemplated.