In order to further characterize Cl- transport of the thin ascending limb of Henle's loop (TAL), we observed the effects of glutaraldehyde on Na+ and Cl- transport in hamster TAL perfused in vitro. We found that 0.1 mol/l glutaraldehyde added either to the lumen or to the bath caused a rapid irreversible reversal of the NaCl diffusion potential. This was mainly accounted for by an inhibition of Cl- permeability (10(-7) cm2 s-1) from 93.51 +/- 8.39 to 14.89 +/- 3.91 (P less than 0.01, n = 9). By contrast, Na+ permeability changed little from 34.18 +/- 3.27 to 26.56 +/- 2.74 (P less than 0.01, n = 6). Glutaraldehyde treatment abolished the halogen-permselectivity of the TAL as determined by the voltage deflection seen upon ionic substitution. Permeabilities for Cl-, Br-, I-, and SCN- relative to Na+ were changed from 3.16 +/- 0.20, 3.22 +/- 0.19, 2.97 +/- 0.26 and 4.36 +/- 0.36 to 0.38 +/- 0.07, 0.35 +/- 0.06, 0.36 +/- 0.07 and 0.58 +/- 0.05, respectively. The effect of glutaraldehyde on the NaCl diffusion potential was dose-dependent in the range from 10(-5) to 10(-1) M. The effect was reversible at concentrations lower than 10(-3) M. Glutaraldehyde did not affect the NaCl diffusion potential of the long-loop descending limb. These observations constitute additional evidence that the mechanism of Cl- transport across the TAL is different from that of Na+ transport. Glutaraldehyde might inhibit Cl- transport in the TAL by cross-linking amino acid residues of the proteins essential for halogen transport across this segment.