Introduction: By measuring the concentration of Rocuronium bromide before and after the ECMO support oxygenator, we can observe whether the circumstances and conditions of extracorporeal circulation have effect on changes of the concentration of the drug. The method of selection is GC/MS, due to its sensitivity and selectivity. Research Objective: Objectives of this thesis are the following: determine the concentration of Rocuronium bromide in venous and arterial samples by using the GC/MS method and compare venous and arterial concentrations of the Rocuronium bromide Materials and Methods: The materials used in this study are plasmas of venous and arterial samples of 5 patients with 10 samples treated with Rocuronium bromide, monosodium phosphate, potassium iodide, ethanol, dichloromethane, acetone, and GC/MS as the analytical method. Results: The GC/MS analysis of the Rocuronium bromide concentration showed the following: if the drug concentration is somewhat higher after 16 and 24 hours in venous blood, it does not differ significantly from other measurements p=0.27. The concentration of the drug in arterial blood takes somewhat lower values after 16 hours, which does not differ significantly compared to other measurements p=0.49. Even though the first three measurements showed greater differences in the concentration of drug in terms of arterial and venous blood, those differences, even if present, are not significant p=0.17-0.69. Conclusion: By analytical method of gas chromatography with mass spectrometry it is possible to measure the concentration of Rocuronium bromide due to the method’s specificity, sensitivity, and selectivity. The differences between venous and arterial samples have no statistical significance, despite lower and expected values in the arterial sample, due to a small number of samples and high variability in measured concentrations.