SummaryThein vivoabsorption of L-valine was studied in segments of the jejunum and ileum using a one-pass perfusion technique in normal suckling (2-week-old), weanling (3-week-old) and adolescent (6-week-old) rats. In both the jejunum and the ileum, rate of absorption (μmoles/h/g wt) of L-valine declined with age. The relationship between absorption rate and luminal concentration suggested the presence both of a saturable carrier mediated (Jmax[C]/Kt+ [C]) and diffusive (KD.[C]) processes for absorption of L-valine. In both the jejunum and the ileum, rates ‘of carrier mediated and of diffusive absorption of L-valine were over 2-fold greater in the suckling than in the adolescent rats. Rates of absorption were not significantly different in the jejunum and ileum at all three age periods. Although values of Jmaxand KDwere greater in segments of the suckling rats, values of the apparent Ktwere similar at all 3 age periods, suggesting that the mechanism (affinity of the carrier macromolecules) of carrier mediated absorption of L-valine did not change with age but the passive permeability of the intestinal mucosa to L-valine and relative number of L-valine absorbing sites (number of carrier macromolecules) decreased per unit weight of intestine with increasing age. Serum and intestinal tissue concentration of L-valine at the end of the perfusion period were greater in the 2-week-old than in the older rats. After parentral injection of [14C]-L-valine rate of secretion of L-valine was several-fold greater in the jejunum and the ileum of the suckling than of the adolescent rats.Speculation The decline in the ability of the small intestine to absorb L-valine (per unit weight) with increasing age appears to be due to a decrease in the passive permeability of the mucosal membrane to L-valine as well as to a relative decrease in the number of absorbing sites (carrier macromolecules) per unit weight of intestine. The decline in the passive permeability suggests a change in the biochemical composition or the ultrastructure of the mucosal membrane during maturation. The decline in the absorbing capacity of the small intestine with age could be due to a relative increase in number of villus epithelial cells not involved in absorption of amino acid; however, it is more likely to be due to a decrease in number of amino acid absorbing sites (carrier macromolecules) per villus epithelial cell.