Bicarbonate transport (JHCO3) was studied in rat proximal convoluted tubules by luminal and peritubular microperfusion, and the effects on tubular bicarbonate transport of selective changes in luminal and peritubular bicarbonate concentrations and of changes in luminal flow rate were evaluated. A pH glass electrode was used to measure [HCO3(-)] and gave results similar to those of a microcalorimetric method. Increasing the tubular and peritubular [HCO3(-)] at constant luminal perfusion rate (10 nl.min-1) augmented JHCO3, but JHCO3 increased more when pH changes were prevented by PCO2 adjustments (constant peritubular pH) than when pH was allowed to rise with the increase in [HCO3(-)] (constant PCO2). Elevation of the tubular HCO3(-) load by raising [HCO3(-)] stimulated JHCO3 more than when the HCO3(-) load was raised by enhancing luminal perfusion rate at constant [HCO3(-)] An increase in PCO2 at constant peritubular pH increased JHCO3. Diamox and benzolamide inhibited JHCO3 at luminal concentrations of 2-4 X 10(-4) M, yet a small but significant fraction of JHCO3 remained intact. Capillary perfusion with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (5 X 10(-4) M) depressed JHCO3 by 70%. Acute changes in luminal and peritubular potassium concentrations (range, 2-6 meq/liter) had no effect on JHCO3, but JHCO3 increased moderately but significantly in severe dietary hypokalemia.