T he effects of anion-transport inhibitors on volume reabsorption, and total CO₂ concentrations were examined by in vivo microperfusion of superficial proximal convoluted tubules of rats. The luminal perfusion solution was a high-chloride, low-bicarbonate solution like that in the in vivo late proximal tubule. The anion-transport inhibitors were only added to the luminal perfusion solutions.In tubules perfused with the control high-chloride solution, the rate of volume reabsorption (J_v) was 2.3±0.2 nl/mm·min (n = 18), and the collected total CO₂ concentration was 4.0±0.3 mM. Furosemide (3 mM) caused a marked reduction in volume reabsorption to 0.8±0.3 nl/mm·min (n = 20) and only a slight increase in the total CO₂ concentration of collected samples of perfusate (7.8±0.5 mM). 0.8 mM acetazolamide caused a more pronounced rise in the collected total CO₂ concentrations to 10.7±0.5 mM but only a slight fall in J_v to 1.7±0.3 nl/mm·min (n = 19). Hence, we inferred that inhibition of carbonic anhydrase only partially accounted for the inhibition of J_v by furosemide. 4-acetamido-4′-iso-thiocyanato-stilbene-2,2′-disulphonic acid (0.1 mM), a well-characterized inhibitor of erythrocyte anion exchange mechanisms, also reduced J_v to 1.6±0.3 nl/mm·min (n = 15) without changing the total CO₂ concentrations of the collected perfusates (3.6±0.4 mM). The effect of 4-acetamido-4′-iso-thiocyanato-stilbene-2,2′-disulphonic acid on volume reabsorption could not be explained by carbonic anhydrase inhibition because there was no increase in the total CO₂ concentration of the collected fluids. Furosemide did not significantly inhibit the rate of tracer glucose efflux out of the tubules, which suggests that the effect of furosemide on volume reabsorption was not a result of some nonspecific depression of active sodium transport. These results are discussed with respect to the possible effects of anion-transport inhibitors on the paracellular shunt pathway, active sodium reabsorption, and neutral sodium chloride transport.