Double-barrel microelectrodes were used to determine membrane voltages and the intracellular pH (pH_i) in primary cultures of cortical collecting duct cells (CCD) grown in the absence of aldosterone. Electrophysiologically, two main cell types were identified. In cell type 1, the apical membrane voltage (V_a) was -60 ± 5 mV. The fractional resistance of the apical membrane (fR_a) was 0.40 ± 0.03, and pHi was 7.21 ± 0.04. Exposure to 50 mM K⁺ on the apical side depolarized V_a by 21 ± 4 mV. When Cl^- was replaced by cyclamate two types of responses were observed: (a) depolarization of V_a by 26 ± 3 mV while pH_i remained unchanged, and (b) no change in V_a. In cell type 2, V_a was -36 ± 5 mV, fR_a was 0.91 ± 0.03 and increasing apical [K⁺] from 5 to 50 mM did not change V_a. Two subpopulations were distinguished by the response of pH_i to lowering apical [Cl^-]. In one of them pH_i increased from 6.99 ± 0.05 to 7.11 ± 0.07. In the other, pH_i was significantly decreased from 7.16 ± 0.08 to 7.03 ± 0.07. These results are compatible with the conclusion that about 50% of the impaled cells type 2 have a Cl^-/HCO^-₃ exchanger at the apical membrane. In summary, two different cell types can be identified electrophysiologically in CCD monolayers. Cell type 1 has the electrical characteristics of principal cells. Cell type 2 resembles the intercalated cells. The cell alkalinization observed in approximately 50% of the cells type 2 in response to Cl^- removal suggests the presence of an apical Cl^-/HCO^-₃ exchanger. Thus, these cells should be the bicarbonate-secreting cells. The remaining cells should correspond to the acid-secreting cells.