In two feeding experiments fattening bulls received on average a daily supplement of 170 or 200 mg Monensin. In a further 8-week experiment the daily Monensin supplement was 0; 500 or 1 000 mg per day. Ergotropic Monensin supplements (experiments 1 and 2) did not change the blood count and the Ca, P and Mg content of blood serum and the activity of AP, AST and LAP in the serum remained unchanged. Net acid base excretion and the content of Na, K and Mg in urine were not significantly influenced either. The influence of 500 mg Monensin per animal and day on the feed intake of animals previously given lower supplements was insignificant. 1 000 mg Monensin per animal and day resulted in a 40% decrease of feed intake and permanent diarrhoea. It was connected with a diminishing of the glucose content in the blood and an increase of net acid base, Na and P excretion in urine. The blood count did not change after the Monensin overdose. In conclusion one can say that the ergotropic Monensin supplement did not change the metabolism parameters.