We developed a new computerized recording system to reduce artificial errors by perfusionists during cardiopulmonary bypass. This system included the efficient functions to record the progress of perfusion flow, perfusion pressure, urine output and temperatures from several sources. Furthermore, it could automatically record considerable data of the "GEM 6 PLUS analyzer" which was developed for monitoring of pH, PCO2, PO2, BE, Ca2+, K+ and hematocrit during cardiopulmonary bypass. We considered it very important that the errors of hematocrits associated with electrolytic concentrations. Therefore, we corrected the hematocrit according to a formula based on our experiments, i.e., HCT = GEMHCT + 0.28 x ([Na+]+[K+]) -44. And also we expect that the "GEM 6 PLUS analyzer" will be improved to measure Na+ value in near future.