Unlike adult rats, which transport biotin preferentially in the jejunum of the small intestine, suckling rats transport biotin preferentially in the ileum. The characteristics and kinetics of the transport process of biotin in the ileum of developing suckling rats and the subsequent maturation of the process in weanling and adult rats are not known and, therefore, are examined in this study. Transport studies were performed in suckling (16-d-old), weanling (25- to 27-d-old), and adult (90-d-old) rats using the everted sac technique. Transport of biotin in the ileum of suckling rats was significantly higher (p less than 0.01) than that in weanling rats, and transport in weanling rats was in turn significantly higher (p less than 0.01) than that in adult rats. In all age groups examined, transport of biotin was found to be Na(+)-dependent and saturable as a function of concentration within the physiologic range (less than 10 microM), but linear at high concentrations. However, the Vmax of the carrier-mediated transport system of biotin showed progressive decrease with maturation (3290 +/- 363, 829 +/- 47, and 463 +/- 19 pmol/g tissue wet wt/15 min in suckling, weanling, and adult rats, respectively), whereas no significant changes were observed in the apparent Km (2.8 +/- 0.6, 2.6 +/- 0.4, and 3.4 +/- 0.4 microM in suckling, weanling, and adult rats, respectively). These findings suggest that maturation is associated with a decrease in the number (and/or activity) but not the affinity of the ileal biotin transport carriers. Transport of biotin by the diffusion process was also found to decrease with maturation (diffusion rates of 144, 86.5, and 40 pmol/g tissue wt/15 min were found for suckling, weanling, and adult rats, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)