1. Influx of neutral and basic amino acids into the mucosal epithelium of the rabbit ileum was measured in the presence of high and low concentrations of Na. Uptake measured at an amino acid concentration of 1 mM varied from 10 to 16 nmole . cm-2 . min-1. Removal of Na inhibited the uptake of short-chain amino acids more than long-chain amino acids. 2. Inhibition of uptake of glycine, alanine, leucine and phenylalanine by a selection of non-radioactive amino acids was found to follow a particular pattern. Long-chain amino acids inhibited the uptake of all test amino acids; short-chain amino acids inhibited preferentially the uptake of glycine. 3. The maximum inhibition serine could cause to the uptake of other amino acids was found to vary. Serine inhibited completely the uptake of glycine but a portion of uptake of long-chain amino acids was found to persist, even in the presence of high concentrations of serine. This is taken as evidence for the presence of an amino acid uptake mechanism having no affinity for serine. 4. The apparent affinities of neutral amino acids for the serino-inhibitable system (system 1) varied from about 0.5 mM (for long-chain amino acids) to about 3 mM (for short-chain amino acids). The total uptake of individual amino acids by system 1 was essentially similar when compared at an amino acid concentration of 1 mM. 5. The serine-resistant uptake of neutral amino acids (system 2) constituted up to two fifths of total uptake for long-chain amino acids, measured at amino acid concentrations of 1mM. The affinities of long-chain amino acids for system 2 is thought to be less than for system 1. Serine appears not to interact with system 2. 6. A second component to serine uptake was found to be related linearly to the concentration of serine in the medium. A similar component may contribute to the total uptake of phenylalanine. The possibility that such a component could arise as a space marker artifact is discussed. 7. An independent kinetic analysis of phenylalanine uptake by rabbit ileal mucosa showed that it could not be accounted for on the basis of a single entry system. However uptake could be described kinetically, assuming two systems of mediated entry to be present. The possible presence of a third non-saturable component to uptake does not affect these conclusions. 8. It is concluded that least two systems exist for the mediated entry of neutral amino acids into rabbit ileal mucosa. This fact should be taken into account in any future mechanistic interpretation of carrier-mediated amino acid transport in the small intestine.