C ortical collecting ducts (CCD) from rabbits treated with deoxycorticosterone (DOC) actively secrete bicarbonate at high rates. To investigate the mechanism of bicarbonate secretion, we measured bicarbonate and chloride transport in CCD from rabbits treated with DOC for 9-24 d. Removal of chloride (replaced with gluconate) from both perfusate and bath inhibited bicarbonate secretion without changing transepithelial voltage. Removal of chloride only from the bath increased bicarbonate secretion, while removal of chloride only from the perfusate inhibited secretion. In contrast to the effect of removing chloride, removal of sodium from both the perfusate and bath (replacement with N-methyl-D-glucamine) did not change the rate of bicarbonate secretion. The rate of bicarbonate secretion equaled the rate of chloride absorption in tubules bathed with 0.1 mM ouabain to inhibit any cation-dependent chloride transport. Under these conditions, chloride absorption occurred against an electrochemical gradient. Removal of bicarbonate from both the perfusate and bath inhibited chloride absorption. Removal of bicarbonate only from the bath inhibited chloride absorption, while removal of bicarbonate from the lumen stimulated chloride absorption. We conclude that CCD from DOC-treated rabbits actively secrete bicarbonate and actively absorb chloride by an electroneutral mechanism involving 1:1 chloride/bicarbonate exchange. The process is independent of sodium.