1. Intracellular recording of membrane potential was made from acinar cells of the isolated mouse parotid gland superfused with physiological salt solutions.2. The mean acinar resting membrane potential was - 68.5 mV during superfusion with Krebs-Henseleit solution. Shift of the superfusion solution to one containing ACh or adrenaline (10(-5)M) always caused a transient hyperpolarization (about 10-15 mV).3. The membrane potential was mainly dependent on the extracellular K concentration ([K](o)). Increasing [K](o) tenfold decreased the membrane potential by 50 mV. This depolarization was not mediated by ACh release from depolarized nerve endings, since it was seen in the presence of atropine (1.4 x 10(-6)M) and not caused by the accompanying reduction in [Na](o) to 40 mM caused only a small depolarization (less than 10 mV).4. When the superfusion solution was shifted, during intracellular recording, from a normal Krebs-Henseleit solution ([K] = 4.7 mM) to a K-free solution, a hyperpolarization of about 8 mV was measured. Reintroduction of the normal K-containing solution after a longer period of K deprivation (30-70 min) resulted in a short-lasting pronounced hyperpolarization (about 20 mV) which could be blocked by Strophanthin-G (10(-3)M).5. In contrast to previous reports, the present findings indicate that the membrane potential of salivary acinar cells is similar, with respect to magnitude and K-dependence, to that of cells of more thoroughly investigated tissues, such as muscle and nerve, and that the membrane Na-K pump is electrogenic, at least when the cells have been loaded with Na.