Studied were the acute and subchronic toxicity of monensin-Na in pigs. The investigations were carried out with elancoban-100, containing 10 per cent monensin-Na. A total of 46 pigs were used, weighing 15 to 60 kg. The preparation was applied individually via the nose or with the feed. It was found that nasally at 5 mg/kg the preparation did not lead to intoxication; rates of 10 and 20 mg/kg proved toxic, and a dose of 30 mg/kg was lethal. Feed containing 300 ppm of monensin-Na given but once led to intoxication and death with part of the animals. Pig tolerated well continuous (33 days) intake of feed that contained the sodium salt of monensin in concentrations of 120 and 240 ppm - no changes in the general status and behaviour were observed, nor were there any deviations in the morphologic and biochemical composition of the blood. It was demonstrated that the low amounts of monensin stimulated the weight gain of pigs. On the other hand, the clinical picture of the intoxication consisted in going off feed, higher pulse and respiration rates, involvement of the nervous system (depression of the nociceptive, eye closure, and ear reflexes, paresis of the hind limbs, and in severe cases - of the forelimbs too), hematuria in most of the animals, higher SGPT activity, and delayed ESR. Morphologically, there were vascular and slightly manifested degenerative changes in the parenchymal organs, red to brown urine in the bladder, strongly enlarged gallbladder, and catarrhal and hemorrhagic gastroenteritis. Histologically, there was granular and fatty degeneration of the kidneys, liver, and heart, hyperemia, perivascular and pericellular edema and degenerative changes in the glial cells of the berebrum, cerebellum, and medulla oblongata.