1. The influx of nineteen amino acids into the heart of the living rat was studied by a method specially devised for experiments under controlled conditions in vivo. 2. When, in separate experiments, the concentration of each amino acid in turn was artificially raised in the circulation, the influx of that amino acid into the heart increased. 3. Our data indicate that at least ten of these amino acids enter the heart in vivo by means of saturable carrier-mediated transport systems. The transport rates conform, at least approximately, to Michaelis kinetics and the transport systems are clearly, in the cases of many amino acids, active, i.e. energy-dependent. 4. The amino acids which were studied had rates of influx into the heart which differed from each other over a range of more than 10 to 1, even when allowances were made for the difference in their concentration in the circulating blood. These differences in influx were not related to such factors as the molecular size of the individual amino acids. 5. The amino acids which have a high influx into the heart are mainly those which are needed either to re-synthesize contractile protein or as oxidizable substrates.