The impact of malnutrition on peptide and amino acid absorption has been studied in the immediate postweaning period. At this time peptide uptake is quantitatively more important than amino acid uptake and the vulnerability of the infant to malnutrition is great. Everted rings of rat jejunum were used to investigate the uptake of the peptide glycylsarcosine (Gly-Sar) and the amino acid L-leucine. The animals had been weaned on to isocaloric diets containing 18% or 4% protein. The rats deprived of protein at this age showed a marked growth disturbance with considerable reduction in gut length in addition to poor weight gain. Mediated influx of Gly-Sar and leucine per centimeter of jejunum was reduced in the malnourished animals: Vmax, 77 ± 7.1 (SEM) and 65 ± 3.6 compared with 85 ± 10.6 and 77 ± 4.4 nmol · min-1· cm-1., respectively. But, when expressed in relation to body weight, the maximal transport capacity showed a marked increase with malnutrition, values being 126 and 111 nmol-1, cm-1.100 g-1body weight compared with 39 and 35 nmol-1. cm-1.100 g-1body weight for Gly-Sar and leucine respectively.