Carbonic anhydrase is found in the cytoplasm and brush border membranes of renal proximal tubular cells. Both the soluble and the membrane-bound enzyme have been assigned roles for the secretion of hydrogen ions into the tubular fluid and hence also for the reabsorption of bicarbonate. Attempts were made to differentiate between the roles of these enzymes for the rate of proximal tubular acidification. Proximal tubules of rats were instilled and perfused with bicarbonate solutions containing carbonic anhydrase inhibitors, especially designed to be impermeable to cell membranes. The acidification rate was measured with an antimony micro-electrode system--the only instantly responding micro-pH electrode. The membrane impermeable inhibitors had no effect on this rate in contrast to acetazolamide, which markedly inhibited the acidification rate when administered intraluminally. It is therefore concluded that the cytoplasmic carbonic anhydrase is the important enzyme for the proximal tubular acidification rate, and hence the rate of bicarbonate reabsorption. The function of the brush border enzyme remains an outstanding problem.