Cl- transport was studied in human nasal epithelium, a predominantly Na+-absorbing proximal airway epithelium. Intracellular Cl- activity (aClc) and the electrical potentials across the apical (Va) and basolateral (Vb) membranes were measured with double-barreled, Cl- -selective microelectrodes to characterize the driving forces for Cl- flow across each membrane. Under control conditions (bilateral Krebs-bicarbonate Ringer), Va was -26.1 +/- 1.2 mV, Vb was -36.2 +/- 1.2 mV, and aCL(c) was 42.7 +/- 2.0 mM (n = 34), indicating that Cl- is near electrochemical equilibrium across the apical membrane but significantly above equilibrium across the basolateral membrane. Reduction of luminal [Cl-] from 120 to 3 mM reduced aClc from 42.7 +/- 4.0 to 27.0 +/- 3.5 mM, depolarized Va, and increased fractional apical membrane resistance (fRa) and transepithelial resistance (Rt). Serosal bumetanide reduced aClc by 10 mM without affecting electrical parameters. Reduction of serosal [Cl-] from 120 to 3 mM resulted in a rapid decrease in Vb, a decrease in fRa and an increase in Rt. Also, serosal [Cl-] reduction led to a slow decrease in aClc rom 45.5 +/- 2.5 to 31.1 +/- 4.2 mM) that could be inhibited by bumetanide. The data are consistent with the following conclusions: 1) Cl- is transported across the apical membrane through a conductive pathway; and 2) Cl- is translocated across the basolateral membrane by an electrically silent bumetanide-sensitive cotransport system and by a minor conductive path.