Developmental aspects of the intestinal transport of biotin were examined in suckling (16 day old) and weanling (24 day old) rats using the everted sac technique. The results were compared with those of adult rats previously reported by us using the same intestinal preparation. Transport of biotin was linear for 20 min of incubation in all age groups. Transport of biotin was significantly (p less than 0.05) lower in the jejunum than the ileum of suckling rats but was not significantly different in the jejunum and the ileum of weanling rats. In adult rats, biotin transport was significantly (p less than 0.01) higher in the jejunum than the ileum. In all age groups, transport of biotin in the jejunum was saturable at low concentrations (less than 10 microM) but linear at high concentrations. The apparent Km and Vmax of the saturable process showed a progressive increase from suckling to weanling to adult rats (apparent Km of 0.63, 2.49, and 3.37 microM; Vmax of 18.3, 44.7, and 124.4 pmol/g.min, respectively). On the other hand, the rate of transport by the nonsaturable process showed a progressive decrease with maturation (143.8, 111.6, and 87.5 pmol/g.min for suckling, weanling, and adult rats, respectively). Transport of biotin in suckling and weanling rats was similar to that of adult rats in that it was Na+-, energy-, and temperature-dependent and inhibited by structural analogues. These results demonstrate that biotin transport undergoes clear maturational changes. These changes include a decrease in the affinity and an increase in the activity (and/or the numbers) of the transport carrier, a decrease in the rate of transport by the nonsaturable process, and a change in the preferential site of transport.