In incompletely confluent Madin Darby canine kidney (MDCK)-cells continuous measurements of the potential difference across the cell membrane (PD) were made with conventional microelectrodes during rapid changes of extracellular chloride concentration. During control conditions mimicking in vivo situation, PD averages -50.3 +/- 0.7 mV. Reduction of extracellular chloride concentration from 122 mmol/l to 64.5 mmol/l depolarizes the cell membrane by +1.8 +/- 0.2 mV while reduction to 16 mmol/l leads to a transient, variable depolarization followed by a hyperpolarization of the cell membrane by -11.8 +/- 1.4 mV. 1 mmol/l anthracene-9-COOH hyperpolarizes the cell membrane by -10.7 +/- 1.0 mV, and abolishes the effect of altered extracellular chloride concentration (-0.6 +/- 0.5 mV), 1 mumol/l diphenylamine-2-carboxylate hyperpolarizes the cell membrane by -11.7 +/- 1.4 mV. 10 mumol/l furosemide hyperpolarize the cell membrane by -11.4 +/- 1.4 mV. Step increases of extracellular potassium concentration from 5.4 to 20 mmol/l depolarize the cell membrane by +14.9 +/- 1.0 mV in the absence of inhibitors, by +24.2 +/- 1.3 mV in the presence of anthracene-9-COOH and by +28.8 +/- 0.7 mV in the presence of furosemide. 10 mumol/l isoproterenol depolarize the cell membrane by +2.4 +/- 0.3 mV and increase the depolarizing effect of reducing extracellular chloride concentration to 64.5 mmol/l (+2.9 +/- 0.4 mV). 1 mumol/l forskolin depolarizes the cell membrane by +5.8 +/- 1.0 mV. In conclusion, chloride conductance of subconfluent MDCK-cells may be small during control conditions, is apparently decreased by anthracene-9-COOH and reduction of extracellular chloride concentration but is enhanced by isoproterenol.