Perfused non working isolated rat heart preparations have been used to study whether alanine (0,89 gl-1) could represent an energy source for the myocardium. Its influence on glucose and lactate metabolism in the heart was also studied. The use of 14C labelled compounds permitted the estimation of the oxidation rate of these three metabolites. The results show that: 1) Alanine was not utilized by the heart. 2) Alanine did not alter myocardial glucose metabolism or lactate oxidation. 3) Alanine lowered lactate uptake by the myocardium which may indicate that alanine influences lactate transport through the cell membranes. One may speculate that in vivo, alanine will not interfere with glucose or lactate metabolism by the heart as, even in acute physiological conditions (exercice, hypoxie, fasting) lactate is always present in blood at higher concentration than alanine.