We followed, within the present case study, 51 children, aged 6 to 11, with attention deficit hyperactive syndrome (ADHS); special biochemical tests were made. The magnesium level was found to be moderately decreased in plasma and erythrocytes and the Mg2+ -ATPase activity was reduced in ADHS children. No essential changes of calcium or sodium homeostasis were detected in cells. The MAGNE-B6 drug, when used in the complex therapy of ADHS, normalizes the magnesium homeostasis, it enhances the large and small motility, attention, psychic stability and EEG parameters, and it reduces anxiety. The method of determination of magnesium in erythrocytes and in blood plasma can be used to detect the deficit of the microelement in patients and to monitor the efficiency of therapy.