The transport of two sugars and three amino acids by proximal and distal intestinal segments of 2-, 5-, 10-, 16-, 22-, 26-, 30-, and 70− to 84-day-old rats was investigated. Labeled α-methyl-D-glucoside and D-galactose were the sugars and labeled L-valine, L-lysine, and glycine were the amino acids used. The sugars were selected because they are thought to share a common transport system in the adult rat. Conversely, the three amino acids were selected because they appear to have separate transport systems in the adult rat. Intracellular accumulation was the transport variable investigated.Transport of both sugars and the three amino acids was maximal in early postnatal life (period I; 2–5 days); however, the patterns of decrease in transport to adult levels showed marked variation. The pattern of decrease for both sugars as well as valine was similar, with about 50% of the total decrease occurring betweenperiods IandII(10–16 days), about 33% betweenperiods IIandIII(22–30 days), and 15–20% betweenperiods IIIandIV(70–80 days). The well established predominance of the proximal intestine in sugar transport was not observed untilperiod III. Lysine showed a relatively small decrease, 10%, betweenperiods IandII, the largest decrease betweenperiods IIandIII, 56.4%, and a decrease betweenperiods IIIandIVnearly twice as large as the decrease of any other compound at the comparable age span. Glycine exhibited approximately 70% of its total decrease betweenperiods IandII. The 15% decrease in glycine transport betweenperiods IIandIIIwas less than one-half that shown by any of the other compounds during the same period.These developmental data give further support to the thesis of separate transport systems for neutral, basic, and glycine-preferring amino acids.SpeculationThis study reveals that protein and carbohydrate digestion end products are maximally transported at or shortly after birth. Further, we believe that significant physiologic data may be obtained from the study of the postnatal developmental patterns of digestive processes