Using the whole-cell patch-clamp technique, we investigated developmental changes in the expression of an acetylcholine- (Ach-) activated Cl–conductance in rat submandibular acinar cells. ACh induced an oscillatory inward current in cells isolated from animals older than 5 weeks, but not in animals less than 2–3 weeks of age. The current/voltage (I/V) relationship of the ACh-induced current was that of an outward rectifier, and the current was inhibited by intracellular BAPTA, a Ca2+ buffer, indicating the current was Ca2+ activated. The ACh-induced current was also blocked in the presence of DPC and SITS, two Cl–current inhibitors in other tissues. Ionomycin mimicked the effect of ACh but in a nonoscillatory fashion. The appearance of the ionomycin-induced currents was also age related, as the current was not observed to occur in animals less than 2–3 weeks old. Since both ACh and ionomycin significantly increase cytosolic [Ca2+] in the acinar cells of young animals, the correlation between the age dependence of the ACh-activated Cl–current and the ionomycin-activated Cl–current responses suggests that the lack of responsiveness observed in the young animals is due to the absence of Ca2+-activated Cl–channels, rather than to a deficiency of a cellular mediator.