Segments of ileum from newborn rabbits were mounted as flat sheets in Lucite chambers and transmural fluxes of Na, Cl, and alanine were measured in the absence of electrochemical potential gradients. In the presence of 140 mM Na, the ileum of the newborn exhibited a serosa-positive electrical PD and a corresponding short-circuit current (Isc) which was markedly enhanced by the addition of D-glucose of L-alanine to the mucosal solution. Alanine-induced increments in Isc were a saturable function of the mucosal alanine concentration. In the presence of Na, alanine was actively absorbed, and the net alanine flux was a saturable function of alanine concentration. When the Na in the bathing solutions was completely replaced by choline, Isc declined to near zero, and the response of Isc to mucosal alanine was abolished. In addition, active amino acid transport was abolished in the absence of Na. In the absence of alanine, the isolated ileum of the newborn actively absorbed Na and Cl, and the algebraic sum of the net movements of these ions accounted for Isc. In the presence of alanine, active Cl transport was abolished and Isc was equal to the net Na absorption. These results indicate that the ileum of the newborn rabbit is similar to that of the adult in its ability to actively absorb Na, Cl, and alanine, but differs from the ileum of the adult by having a greater passive permeability to ions and amino acid, and differs particularly with regard to the effect of mucosal alanine on transmural ion transport.