The gastric enterochromaffin-like (ECL) cell secretes histamine in response to secretagogues (gastrin, acetylcholine) by calcium signaling-dependent exocytosis of intracellular vacuoles containing the hormone. ECL cells were isolated from rat fundic gastric mucosa by elutriation and density-gradient centrifugation. Currents across the plasma membrane were measured using whole cell patchclamp methods. These cells had a low conductance of 0.5 nS and resting potential of -50 mV. Depolarization activated a K+ current that was blocked by Ba2+. Steady-state current in absence of K+ was due to Cl- because of the magnitude of the reversal potential and the effects of Cl- removal. Stimulation of secretion by gastrin, cholecystokinin octapeptide (CCK-8), and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate activated the Cl- conductance with a time course similar to that of histamine release. Therefore the ECL cell maintains a high resting potential, largely due to K+ currents, and stimulation of secretion activates a Cl- current, perhaps deriving from the membrane of the secretory granule that fuses with the plasma membrane. The depolarization that ensues may activate the K+ current to maintain the membrane potential during exocytosis.