We had previously shown that neonatal rabbit tubules have a lower chloride permeability, but comparable mannitol permeability compared to adult proximal tubules. The surprising finding of lower chloride permeability in neonatal proximals compared to adults, impacts net chloride transport in this segment, which reabsorbs 60% of the filtered chloride in adults. However, this maturational difference in chloride permeability may not be applicable to other species. The present in vitro microperfusion study directly examined the chloride and mannitol permeability using in vitro perfused rat proximal tubules during postnatal maturation. While there was no maturational change in mannitol permeability, chloride permeability was 6.3±1.3 x 10^-5 cm/sec in neonatal rat proximal convoluted tubule and 16.1±2.3 x 10^-5 cm/sec in adult rat proximal convoluted tubule, p<0.01. There was also a maturational increase in chloride permeability in the rat proximal straight tubule (5.1±0.6 x 10^-5 cm/sec vs 9.3±0.6 x 10^-5 cm/sec, p<0.01). There was no maturational change in P_HCO3/P_Cl in the rat PST and PCT or in the P_Na/P_Cl in the proximal straight tubule; however, there was a significant maturational decrease in proximal convoluted tubule P_Na/P_Cl with postnatal development (1.31±0.12 in neonates vs 0.75±0.06 in adults, p<0.001). There was no difference in the transepithelial resistance measured by current injection and cable analysis in the proximal convoluted tubule, but there was a maturational decrease in the proximal straight tubule (7.2± 0.8 vs 4.6±0.1 ^. cm², p<0.05). These studies demonstrate that there are maturational changes in the rat paracellular pathway that impacts net NaCl transport during development.