Active transport potentials were studied across early loops of rat proximal tubule during luminal perfusion and peritubular superfusion with HCO3- Ringer's solution of identical ionic composition. From the effects of the carbonic anhydrase inhibitor acetazolamide and of ouabain it is concluded 1. that the lumen-positive active transport potential indicates an excess of active H+ secretion/HCO3- absorption over active Na+ absorption and 2. that the lumen-negative active transport potential, which develops in the presence of glucose (and/or aminoacids) in the tubular lumen, indicates stimulation of active Na+ absorption. Ouabain did not abolish the lumen-positive potential difference suggesting that active H+/HCO3- transport and active Na+ transport may be to some extent independent. Among the diuretics tested the mercurial diuretic mersalyl acted primarily on Na+ transport, and furosemide acted on HCO3- transport, whereas the effect of ethacrynic acid appeared to be unspecific.