The rate of unidirectional uptake, v, of L-[4,5-3H]leucine was studied in 17 isolated dog brains by means of an indicator-dilution technique using 22Na as the intravascular reference. The arterial leucine concentration was varied in increments by adding unlabeled leucine to the blood and v was determined after each change. The valine and isoleucine concentrations were varied independently to permit evaluation of their effect on leucine transport. A preliminary analysis indicated that both valine and isoleucine are competitive inhibitors. Therefore, all data were fitted to an equation that describes Michaelis-Menten kinetics in the presence of two competitive inhibitors. These calculations yielded an apparent Km for leucine transport of 1.58 mM plus or minus .28 SE, a V max of 0.323 mumol/g per min plus or minus .035 SE, and an apparent K i for inhibition of leucine transport of 1.76 mM plus or minus .34 SE for valine and 0.73 mM plus or minus .14 SE for isoleucine. In four isolated brains perfused with blood having a constant leucine level, indicator-dilution injections were made before, and at 1, 5, and 10 min after the start of perfusion with anoxic blood. These findings showed that, unlike glucose transport (Brain Res. 67: 307-316, 1974), the rate of lucine transport is unaffected by 10 min of anoxia.