SUMMARY1. Transepithelial potential difference (PDte) of proximal tubules was measured in rats under control conditions (C), and mannitol‐saline and saline extracellular fluid volume expansion (MVE, SVE, respectively) under conditions of normal net lumen to basal sodium transport.2. PDtewas measured in kidneys bathed with Hartmann's solution or covered with mineral oil under both volume‐expanded conditions together with their controls.3. PDtewas significantly lower in kidneys bathed with Hartmann's solution than in those covered with oil.4. In MVE rats, with mineral oil covering the kidneys, PDte(expressed as mean and s.e.m.) was for the control 2.20 ± 0.05 (n= 45) mV and MVE 1.97 ± 0.04 (n= 36) mV, lumen positive, a significant reduction of 10% (P<0.001). In SVE rats, with mineral oil covering the kidneys, PDtewas for C = 2.42 ± 0.05 (n= 74) mV and SVE = 1.93 ± 0.03 (n= 67) mV, a significant reduction (P<0.001) of 20%.5. According to thermodynamic considerations, neither of these changes is sufficient to explain the 50% inhibition of Na transport measured previously during MVE and SVE with autologous tubular fluid. The present results offer further evidence supporting the idea that the inhibition of Na transport during MVE and SVE is largely due to inhibition of the active Na transporti