Application of 10 nM Epidermal Growth Factor (EGF) to single EGFR-T17 fibroblasts induced a marked hyperpolarization that could last for tens of minutes; in many cases the first transient was followed by a series of oscillations of the membrane potential. The outward current responsible for the hyperpolarizing response could be recorded simultaneously to an increase in the intracellular calcium concentration, as measured with the fluorescent indicator fura-2. The conductance was nearly linear in the voltage range from -100 to +50 mV. While the EGF-induced current had many characteristics of a K+ current and was strongly reduced by 50 nM charybdotoxin (ChTx), its reversal potential was apparently more negative than the potassium equilibrium potential (VK). The application of 2 microM ouabain prior to EGF stimulation produced responses that were similar to those obtained without ouabain; however, under these conditions the EGF-induced current showed a reversal potential of -96.6 +/- 3.2 mV, very close to VK. Simultaneous application of both 2 microM ouabain and 50 nM ChTx completely abolished the response. It can be concluded that the response to EGF stimulation in EGFR-T17 cells consists of two components: the first is a current carried through Ca(2+)-activated K+ channels; the second is due to the acceleration of the operation of the Na+/K(+)-ATPase.