Developmental changes in cystine transport by the canine kidney were examined both in vivo and in vitro. Renal clearance studies indicated that cystine was one of the more incompletely reabsorbed amino acids at birth, but its reabsorption approaches adult levels by 21 days. Concomitantly, cystine uptake by isolated renal cortical tubule fragments from immature dogs was slower than that by renal tubules from adult dogs. Both age groups rapidly metabolized the transported cystine. This metabolism was principally to cysteine, but also small amounts of reduced glutathione were formed from the transported cystine. Concentration dependence studies indicated two transport systems for cystine uptake in both the immature and the adult dog. Both transport systems in the 1-wk-old dog had a somewhat greater affinity for cystine than the corresponding system in the adult, but this was offset by the markedly lower maximal transport rates for these systems in the 1-wk-old dog. The high affinity system was inhibited by lysine in tubules from both age groups. In the dog, the rise in the tubular reabsorption of cystine with maturation could, in part, be explained by an increase in the number of transport sites for cystine.