A micropuncture study was performed on the bullfrog kidney proximal tubular cells utilizing double-barreled ion-selective microelectrodes. The intracellular of Na+, K+, Cl-, HCO3(-) and pH were determined to be 21.6 mEq/L, 67.4 mEq/L, 9.9 mEq/L, 20.2 mEq/L, and 7.49 pH units, respectively. In the extracellular fluid the following activities were found: Na+, 87.4 mEq/L; K+, 2.64 mEq/L; Cl-, 72.5 mEq/L; HCO3(-), 17.9 mEq/L; and pH, 7.66. The membrane potential difference was 68.4 mV and 60.4 mV across the peritubular and brush borders, respectively. The electrochemical potential differences across the individual borders of the proximal tubule cells were separately calculated by setting the intracellular level of both electrical and chemical potentials at zero for convenience. From these analyses, the following interpretations are made. (1) In the net reabsorption of Na+, luminal Na+ enters the cell along a 95-mV gradient across the luminal border and is pumped out to the interstitium against a 104 mV gradient. In the reabsorption of bicarbonate, an uphill pump of about 69 mV (about 70% of the Na+ entry gradient) must exist on the luminal border, of which about 55 mV (80% of the bicarbonate gradient) is accounted for by the H+ secretory pump. (2) In the net reabsorption of K+, a significant K+ uptake pump must exist on the luminal border in addition to the powerful peritubular Na+-K+ exchange pump. The reabsorption of Cl- by the epithelium may take place in two ways: (a) transmembrane transport involving an uphill step of several millivolts, and (b) paracellular leakage through the tight junction. It is thought that the Na+ pump located on the basolateral border of the proximal tubule cell plays a primary role in the regulation of the movement of other ions and water. The regulatory mechanisms of these substances may involve some electrochemical feedback mechanism that works across the proximal tubular epithelium.