Techniques were developed for the measurement of intracellular potentials and potassium activities in rat proximal tubule cells using double barreled K+ liquid-ion-exchanger microelectrodes. After obtaining measurements of stable and reliable control values, the effects of K+ depletion and metabolic and respiratory acidosis on the intracellular potential and K+ activity in rat kidney proximal tubular cells were determined. At a peritubular membrane potential of -66.3 +/- 1.3 mV (mean +/- SE), intracellular K+ activity was 65.9 +/- 2.0 mEq/liter in the control rats. In metabolic acidosis [70 mg NH4Cl/100 g body wt) the peritubular membrane potential was significantly reduced to -47.5 +/- 1.9 mV, and cellular K+ activity to 53.5 +/- 2.0 mEq/liter. In contrast, in respiratory acidosis (15% CO2) the peritubular membrane potential was significantly lowered to -46.1 +/- 1.39 mV, but the cellular K+ activity was maintained at an almost unchanged level of 63.7 +/- 1.9 mEq/liter. In K+ depleted animals (6 weeks on low K+ diet), the peritubular membrane potential was significantly higher than in control animals, -74.8 +/- 2.1 mV, and cellular K+ activity was moderately but significantly reduced to 58.1 +/- 2.7 mEq/liter, Under all conditions studied, cellular K+ was above electrochemical equilibrium. Consequently, an active mechanism for cellular K+ accumulation must exist at one or both cell membranes. Furthermore, peritubular HCO3- appears to be an important factor in maintaining normal K+ distribution across the basolateral cell membrane.