In this study we investigated that the effect of aminoglycoside on the serum magnesium level. Therefore 3 mg/kg/day gentamicin was given during 10 days. Serum and urine magnesium levels were studied with the method of Titan yellow. 60 per of the patients were women and 40 per were men. Their ages were between 20 and 82. We observed that serum magnesium level began to fall down from the early period of our study. Serum magnesium level was found explicitly lower before the treatment. At the end of the treatment discharge of magnesium with urine found higher than the initial valve. We didn't observe the symptoms of hypomagnesemia in any of the patients. We thought that hypomagnesemia developed in the patients who were given aminoglycoside as a result of increasing magnesium lost. While the lowest effective dose is applied, it ran not reach the level of giving hypomagnesemia symptom but while aminoglycoside is applied on the patients under risk. We agree that developing of symptoms belonging to hypomagnesemia should not be ignored and if possible the level of serum magnesium should be checked.