From each of 2 tetanic herds and 1 control herd 20 cows were randomly selected to determine serum levels of Mg, Ca, inorganic P, Na, K, Cl-, total protein, urea, GOT-activity and blood-glucose level. In rumen fluid the number of ciliates, ammonia, level, pH and total acidity were examined. Haematological investigations were carried out. Moreover dry matter and height of the herbage were measured, and meteorological measurements were utilized. The investigations were carried out before grazing and than on the 4th, 8th, 12th, and 16th day of grazing. Symptoms of metabolic disturbances and alcalic indigestion were found to be more distinct in the tetanic herds than in control herd. The method of MgO-pulverization on pastures was found to be useless. On the other hand, MgO-food-supplementation according to generally known methods was found to be of high usefulness. MgO-treatment at a dose of 50 g per cow and day in a period of 14 days before grazing effectively prevented the outbreak of hypomagnesaemia in spite of such tetanogenic factors as unfavourable weather, insufficience of dry matter in the herbage and subclinical alcalic indigestion with disturbed nitrogen metabolism.