The early proximal tubule preferentially reabsorbs organic solutes and bicarbonate over chloride ions, resulting in a luminal fluid with a higher chloride concentration than that in blood. From this late proximal tubular fluid, one-half of NaCl reabsorption by the adult proximal tubule is active and transcellular and one-half is passive and paracellular. The purpose of the present in vitro microperfusion study was to determine the characteristics of passive chloride transport and permeability properties of the adult and neonatal proximal straight tubules (PST). In tubules perfused with a late proximal tubular fluid, net passive chloride flux was 131.7 +/- 37.7 pmol x mm(-1) x min(-1) in adult tubules and -17.1 +/- 23.3 pmol x mm(-1) x min(-1) in neonatal proximal tubules (P < 0.01). Chloride permeability was 10.94 +/- 5.21 x 10(-5) cm/s in adult proximal tubules and -1.26 +/- 1.84 x 10(-5) cm/s in neonatal proximal tubules (P < 0.05). Thus neonatal PST have a chloride permeability not different from zero and have no net passive chloride transport. Bicarbonate permeability is also less in neonates than adults in this segment (-0.07 +/- 0.03 x 10(-5) vs. 0.93 +/- 0.27 x 10(-5) cm/s, P < 0.01). Neonatal PST have higher sodium-to chloride and bicarbonate-to-chloride permeability ratios than adult PST. However, mannitol and sucrose permeabilities were not different in adult proximal tubules and neonatal PST. Transepithelial resistance was measured using current injection and cable analysis. The resistance was 6.7 +/- 0.7 Omega x cm(2) in adult tubules and 11.3 +/- 1.4 Omega x cm(2) in neonatal PST (P < 0.01). In conclusion, there are significant maturational changes in the characteristics of the PST paracellular pathway affecting transport in this nephron segment.