T he mechanism by which the kidney reabsorbs sodium bicarbonate could be a result of (a) H+ secretion, (b) direct HCO3- reabsorption, or (c) a combination of both processes. Most of the studies which have supported the H+ secretory theory have involved the assumption that tubular fluid and arterial PCO2 were equal. We have utilized a new PCO2 microelectrode to directly determine in situ PCO2 of tubular fluid and stellate vessel blood in the cortex of the rat kidney during control conditions and after alterations in acid-base status. In 21 control rats, proximal tubular fluid PCO2 exceeded systemic arterial PCO2 (deltaCO2) by 25.9 +/- 0.92 mm Hg (P less than 0.001). The values obtained for both distal tubular fluid and stellate vessel blood were not significantly different from proximal tubular PCO2. Evaluation of PCO2 in the proximal tubules of Munich-Wistar rats did not reveal evidence for a declining profile for PCO2 along the length of the nephron. When proximal bicarbonate reabsorption was increased or decreased acutely by alterations in acid-base status, deltaPCO2 changed in paralle. Furthermore, benzolamide administration significantly reduced deltaPCO2. We conclude: (a) that the PCO2 in tubular fluid is significantly greater than systemic arterial PCO2, (b) that there is no tendency for the observed PCO2 to fall along the proximal tubule, (c) the mean PCO2 in the proximal and distal tubules as well as the stellate vessle is not significantly different, thereby rendering the concept of a "diffusion barrier" for CO2 in the proximal tubule unlikely, and (d) the level of renal cortical PCO2 appears to vary directly with the magnitude of bicarbonate reabsorption.