A clinical and morphological study was carried out on mass scale poisoning of heifers with methylparathion. Out of a total of 80 animals in a herd 13 heifers died or were forcedly slaughtered within several hours to three days, and other 9 heifers were cured after diagnosing severe methylparathion poisoning. The presence of the preparation was demonstrated chemically in the stomach and intestinal content of the affected animals. The precise lethal dose, however, was not established owing the outbreak of spontaneous poisoning. On the base of literature data and personal investigations it is believed that in the case described the intake of poisonous matter largely surpassed the lethal dose for cattle (20 mg/kg body mass), particularly on the occasion of a peracute course of the disease. Clinically, the intoxication ran its course with troubles on the part of the central and the vegetative nervous systems. Morphologically, there were oedema of the brain and the meninges, oedema and emphysema of the lungs, degeneration processes in the liver and the other parenchymal organs, oedema and hemorrhagic infiltrations in the wall of the rumen, abomasum, and proximal portions of the intestine, and sporadic or numerous hemorrhages in the organs referred to, particularly on the epi- and endo cardium.