The kinetics of the efflux of exogenous amino acids from brain slices were obtained from published data. Efflux is by 3 concurrent processes, each representable to a first approximation by an exponential decay (first-order rate process): (1) initial rapid efflux, which contributes measurably only during the initial 2–5 min; (2) the first efflux process, which contributes measurably for up to 90 min depending on the amino acid and preparation; and (3) the second efflux process, which contributes for the duration of the experiment. Initial rapid efflux is due to loss from the surface and accessible extracellular spaces. The first efflux process apparently represents exit of intracellular substrate, but may conceivably represent loss from less accessible extracellular spaces. The second efflux process is clearly due to outflow from intracellular compartments. The second efflux process has both a saturable (Michaelis-Menten) component and an unsaturable component. At low concentrations of substrate in tissue, 60–90% of the flux by this process is through its saturable component. The overall rate equation for efflux is v = k rC r + k 1C 1 + [V max/(C 2 + K t) + k 2]C 2. The parameters, k 1, k 2, V max and K t, depend greatly on the amino acid and on the tissue. Consequently, curves of concentration vs. time for different systems may appear to be qualitatively different. The rate law requires that exogenous amino acids be in 3 kinetically distinct pools, one for each efflux process. Because the distribution of amino acid among the 3 pools, C r, C 1 and C 2, can differ for the same overall concentration in tissue, the efflux curve depends on how any given tissue concentration was achieved.