The effect of mechanical work load on net proteolysis, amino acid catabolism and ammonia production was studied in isolated perfused beating or K+-arrested hearts. Net proteolysis was about 16 mumol/g dry wt. during 1h and was not affected by the mechanical work. The combined catabolic rate of the major amino acids was 7.1 mumol/g dry wt. in the beating heart and 2.1 mumol/g dry wt. in the arrested heart during the 1 h experimental period. The main differences lay in the deamination of aspartate plus glutamate, which was inhibited by 60% during low energy consumption, and in net alanine synthesis, which was increased by 94%. The ammonia production plus its conversion into amide nitrogen was 9.2 and 3.4 mumol/g dry wt. in the beating and arrested heart respectively during 1 h. The decrease in the total adenine nucleotide pool during the 1 h perfusion was very low, 1.0 and 0.5 mumol/g dry wt. in the beating and arrested hearts respectively, and did not contribute significantly to ammonia production. Thus ammonia production is dependent on the cellular energy state, whereas net proteolysis is not. The maximal capacities of the purine nucleotide cycle and the glutamate dehydrogenase reaction towards deamination were much higher than the observed ammonia-production rates. The anaplerotic role of amino acid catabolism in the myocardium is discussed.