The postoperative period is associated with the biochemical features of Non-Thyroidal-Illness (NTI). Usually, there is a marked decrease of T3 due to a decreased 5`dejodinase (a selenoprotein) activity. Few studies have shown that selenium (Se) substitution may alter the low T3-state in severely ill patients. However, there are no data evaluating Se-substitution in the postoperative state. Therefore, we investigated prospectively the effect of parenteral selenium substitution in patients undergoing CPB. 20 patients received 500µg selenium i.v. as a bolus and 1000µg Se i.v./24h from the immediate postoperative period up to 4 days. 23 patients served as a control group. Selenium administration was associated with an increase in serum selenium-concentrations to the upper normal limit. Throughout the observation period (mean serum selenium concentrations ±SD increased from 0,60±0,28µmol/l to 1,29±0,15 µmol/l).Whereas patients in the control group were well below normal limits (below 0,71±0,36µmol/l; p<0,01 controls vs. Se-group). However, glutathione peroxidase activity (GPx) was not significancy different from the controls (2,92±1,7 vs. 1,34±0,4 nmol NADPH/min/mg day 4 postoperative). T3 levels after surgery were not altered by the Se administration (T3 Se-group 1,26±0,30 nmol/l vs. T3 controls 1,08±0,27 nmol/l on day 1 after CPB). Also TSH, T4, fT4 and rT3 were not influenced by selenium. Moreover, there was no difference in mortality and postoperative cardiac parameters in either group. In conclusion: Selenium administration to levels in the upper normal range was not associated with alterations in the severity of NTI during the postoperative period after CPB.