Adrenaline myocarditis in rats was accompanied by a decrease in relative activity of pyruvate kinase from hyaloplasm of heart muscle, especially pronounced within 6-13 hrs after adrenaline administration. This phenomenon was probably due to an increase in content of protein in hyaloplasm, but not to an alteration in the enzyme activity. This conclusion was confirmed by studies of some properties of pyruvate kinase. Under these pathological conditions the Km of phosphoenol pyruvate was of the same order of magnitude as in normal state; 1-phenylalanine was characterized as an inhibitor of non-competitive type tomards phosphoenol pyruvate, and palmitate did not exhibit the inhibitory effect on pyruvate kinase as it was found in normal state. The effectiveness of the enzyme inhibition by L-phenylalanine, protection of the enzyme against this amino acid effect by L-alpha-alanine, L-serine and L-cysteine, and also the degree of activation of pyruvate kinase by the amino acids under adrenaline myocarditis were similar to those phenomena observed in normal state.