Proximal straight tubules (PST) from both neonatal and hypothyroid adult rabbits have a lower rate of passive volume absorption when perfused with a high chloride solution simulating late proximal tubular fluid than adult rabbit PST. We hypothesized that the maturational increase in serum thyroid hormone levels mediates the developmental changes in PST paracellular permeability. Neonatal tubules had lower chloride permeability, higher transepithelial resistance, but comparable mannitol permeability compared to adult PST. The present in vitro microperfusion study directly examined whether thyroid hormone affects passive solute flux and if thyroid hormone could explain the developmental changes in PST paracellular permeability. Passive chloride transport was 62.1±4.5, 23.1±7.7 and 111.6±5.6 pmol/mm^.min, in PST from euthyroid, hypothyroid and hypothyroid animals that received thyroid treatment, respectively (control different from hypothyroid and thyroid treatment at p<0.05). This was due to a thyroid hormone mediated change in chloride permeability. Mannitol permeability was 3.65+1.03, -0.19+0.72 and 3.60+1.12 x 10^-6 cm/sec in PST from euthyroid animals, hypothyroid animals and hypothyroid rabbits that received thyroid replacement, respectively (p<0.05, hypothyroid vs euthyroid and thyroid replacement). We demonstrate that PST from hypothyroid animals have a higher passive P_Na/P_Cl and P_HCO3/P_Cl than euthyroid controls. Finally, we examined if these changes in permeability were paralleled with a change in PST paracellular resistance. Resistance was measured by current injection and cable analysis. The resistance in PST from hypothyroid rabbits was 6.3±0.8 ^.cm², which was not different than control of 4.8±0.7 ^.cm², or 7.0±0.7 ^.cm² in hypothyroid animals that received thyroid replacement. Therefore, the maturational increase in thyroid hormone levels do not fully explain the developmental changes in the paracellular pathway.