The causes of variable laboratory results as well as the variable electrolyte's serum concentration may be numerous but the most are preanalytical. This research investigated whether the serum electrolyte's concentrations change with time and different storage conditions in tubes with and without the separation gel. In this study it was used 40 blood samples taken in the clinical laboratory of University Hospital Center Split . Twenty samples were taken in the tubes without separation gel and the remaining twenty in a tubes with separation gel. The samples were centrifuged after coagulation process and the concentrations of electrolytes sodium, potassium and chloride were measured in all samples immediately using indirect potentiometry with ion selective electrodes for sodium, potassium and chlorides. The subsequent measurements were performed after two and four hours in samples stored at room temperature. The following two measurements are made after 24 and 48 hours of storage at + 4 ˚ C. The results of all four delayed measurements were compared with the results of initial measurments in order to observe if there is a change in concentration. The aim of the study was to observe whether the electrolyte concentrations change with a time delay and different storage conditions in tubes with and without the separation gel. The concentrations of sodium in the tubes with separation gel have changed in relation to the initial measurments in all time points, showing a slight increase. The concentrations of sodium in the tubes without separation gel show a slight increase after 4 and 48 hours after the initial measurement in relation to the measured initial concentration, while the concentration measured 2h and 24 h after the initial measurement were unaffected. The concentration of potassium after 48h in the tubes with separation gel shows a slight increase in relation to the initial concentration while the concentrations after the 2,4 and 24h were unchanged in relation to the initial. The concentrations of potassium measured 2 and 4 hours after initial measurment in the tubes without separation gel were unchanged compared to the initial while the results measured after 24 and 48 hours show a slight increase. The concentration of chloride measured 2 hours after initial measurment in the tubes with separation gel was unchanged in relation to the initial concentration, while the concentrations measured after 4,24 and48 h show a slight increase in relation to the initial. Concentrations of chloride in the tubes without the separation gel were unchanged compared to initial in all time points. This small study examined if there is a change of electrolyte's concentrations measured in laboratory conditions which relates to the open samples in environmental conditions (variable room temperature) and the storage of closed samples in cooling conditions at the end of daily work. The study has a number of limitations that are primarily related to the small number of samples and measurements The coeficients of analytical variation were not tested despite the fact that analitical variation could affect the measured results. Therefore, the observed results should certainly be examined by the the extended study which would examine the stability of electrolyte in a particular type of sampling tubes in order to get more precise data which would help to correct the common laboratory procedures.