1. The kinetic parameters (Km, Vmax and Kd) of L-glutamine, L-glutamate and L-lysine uptake by isolated red blood cells in fed and 24 hr starved rats have been determined. 2. L-Lysine and L-glutamine uptake was best fitted by a two transport component: a saturable component and a diffusion one. 3. Starvation brought about important decreases in the Km and Vmax for both L-lysine and L-glutamine uptake. 4. The Kd for L-glutamine showed a significant increase whereas that corresponding to L-lysine did not change by starvation. 5. L-Glutamate uptake adjusted to diffusion kinetics, with a Kd which did not change due to starvation. 6. It is concluded that the amino acid uptake showed specific regulation by starvation. 7. The mechanism involved is not dependent on protein synthesis--given the unnucleated nature of mammal red cells. 8. The magnitude of the changes observed in the uptake kinetic parameters may account for the extent of the blood amino acid pool changes as those produced in vivo over physiological limits.