Madin-Darby canine kidney (MDCK) cells form monolayers with properties resembling natural transporting epithelia. In this paper we first summarize briefly information on the electrical properties of the whole monolayer and on the paracellular permeation route, and then concentrate on the electrical properties of the cells themselves as studied with intracellular microelectrodes of high resistance (approximately 100 M omega). The membrane resistance is -40.5 + 15 (120) mV, yet the shape of the distribution curve shows that a group of artifactually low values is displacing the average, so the actual potential may be close to -50 mV. The electrical resistance is 61.6 +/- 6.3 (92) M omega and the capacity is 45.1 +/- 2.9 (63) pF. The relationship between resistance and capacity was 1981 +/- 177 (61) omega. microF . No electrical coupling was found in 20 pairs of neighboring cells. The current/voltage curve shows a marked asymmetry, and in some cells, voltage becomes dependent on time for large, depolarizing current pulses. This phenomenon can be accentuated by removing Na+ or adding amiloride.