During the tests carried out with rats the organic complex magnesium gluconate was examined for its bioavailability and its stress-reducing properties after oral application. The increase of magnesium concentration in serum achieved by means of magnesium gluconate turned out to be higher than the increase of concentration due to the administration of magnesium chloride. Just like other organic magnesium complexes, magnesium gluconate, administered for three days via feed to rats under stress conditions of immobilization, the substance causes a significant reduction in the increase in catecholamine and corticosterone concentration in serum. Regarding the reduction of stress in rats, the optimal concentration in feed is between 500 and 1000 mg of magnesium as magnesium gluconate per kg.