Total and ionized magnesium in the blood serum and daily urine were determined before (baseline values) and on days 30, 60, and 105 of the experiment with a 105 day of isolation and confinement (Mars-105). Magnesium in hair was investigated before (baseline values) and on day 105 of the experiment. The investigations were performed using atomic emission spectrometry with inductively coupled argon plasma. Changes in the magnesium level were the largest during the first 30 days of the experiment. The reduction of serum magnesium level was mainly due to the fall of the ionized fraction. In the body, magnesium is controlled by the ion-regulatory function of the kidneys and varies between individuals. The levels of ionized magnesium in the blood serum and those excreted in daily urine can serve as indicators of stress resistivity.