Because renal function in newborns is immature, the pharmacokinetics of drugs administered to neonates vary significantly from adult patients. The establishment of drug transport systems is a key process in the functional maturation of the nephron. However, a thorough examination of the expression of the main drug transporters in the kidney throughout all stages of development (embryonic, postnatal, and mature) has yet to be carried out, and the functional (physiological) impact is not well understood. Using time-series microarray data, we analyzed the temporal behavior of mRNA levels for a wide range of SLC and ABC transporters in the rodent kidney throughout a developmental time series. We find dynamic increases between the postnatal and mature stages of development for a number of transporters, including the proximal tubule-specific drug and organic anion transporters (OATs) OAT1 (SLC22a6) and OAT3 (SLC22a8). The OATs are the major multispecific basolateral drug, toxin, and metabolite transporters in the proximal tubule responsible for handling of many drugs, as well as the prototypical OAT substrate para-aminohippurate (PAH). We therefore performed specific in vivo pharmacokinetic analysis of the transport of PAH in postnatal and maturing rodent kidney. We show that there is a 4-fold increase in PAH clearance during this period. Clearance studies in Oat1 and Oat3 knockouts confirm that, as in the adult, Oat1 is the principle transporter of PAH in the postnatal kidney. The substantial differences observed supports the need for better understanding of pharmacokinetics in the newborn and juvenile kidney compared with the adult kidney at the basic and clinical level.