α-Methyl-D-glucoside (AMG) uptake was examined in isolated renal cortical tubules from newborn, 3-month-old, and adult dogs. All three age groups demonstrated active sugar transport. The initial rate of AMG uptake was similar in the 3-month-old and adult tubules which was twice that of the newborn. At steadystate, the adult and newborn tubules had achieved a similar intracellular AMG concentration which was 45% greater than that of the 3-month-old. Determination of the flux constants of these uptake patterns revealed that there was an age-dependent increase in both the net flux and the fractional influx rate constant. However, the 3-month-old had the highest fractional efflux rate constant and the newborn the lowest value with the adult in between. Kinetic analysis of AMG uptake showed a single saturable transport system for each age group. The newborn and adult had similar Kmvalues but the 3-month-old had a value that was 60% higher. The 3-month-old tubules had the highest Vmaxand the newborn tubules the lowest with the adult value in between. AMG uptake by tubules from each age group demonstrated a similar pattern of inhibition in a low sodium buffer and by glucose and phlorizin. This indicated that, aside from kinetic changes with maturation, the saturable transport system for AMG is similar in each age group.