Patch-clamp whole-cell current recordings under voltage-clamp conditions were carried out on isolated mouse exorbital lacrimal acinar cells. Acetylcholine evoked outward current at a membrane potential of -20 mV whereas an inward current was observed at -80 mV. The outward current is due to the well-known calcium-activated K+ channels whereas the inward current was C1- dependent. The acetylcholine-evoked C1- current was abolished when the intracellular Ca2+ concentration was clamped at very low levels by a high intracellular EGTA concentration. Acetylcholine therefore activates a Ca2+-dependent C1-conductance in mouse lacrimal acinar cells.