The effects of the anion transport inhibitor DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulphonate) on fluid reabsorption by the rat proximal convoluted tubule were investigated by the stationary microperfusion (split-drop) method. Addition of 10(-5) mol 1-1 DIDS to luminal and capillary perfusates in the presence of peritubular bicarbonate (25 mmol 1-1) increased the reabsorptive half-time (t 1/2) from 35.1 +/- 4.5 to 57.0 +/- 6.2 s, corresponding to a decrease in fluid reabsorption (Jv) from 1.33 +/- 0.06 to 0.89 +/- 0.09 nl min-1 mm-1. Omission of bicarbonate per se from the peritubular capillaries reduced Jv to 0.41 +/- 0.03 nl min-1 mm-1 and this reabsorptive flux was completely inhibited by the addition of 2 mmol 1-1 NaCN to both the luminal and peritubular perfusates. In the absence of bicarbonate, the addition of DIDS at either 10(-4) or 10(-5) mol 1-1 to both lumen and capillary perfusates had no significant effect on either t 1/2 or Jv. These results show that the inhibitory action of DIDS on fluid reabsorption is dependent on the presence of bicarbonate. However, the proximal tubule also reabsorbs water in the absence of bicarbonate by a mechanism that is dependent on cellular metabolism, but is insensitive to DIDS at the concentrations used in this study.