1. We used the whole-cell configuration of the patch clamp technique to examine the different macroscopic Cl- currents present in single rat parotid acinar cells. 2. Cell swelling produced by negative osmotic pressure (hypotonic bath solutions) induced a large outwardly rectifying Cl- current with little or no time and voltage dependence. In contrast, an increase in intracellular [Ca2+] induced by ionomycin activated Cl- currents with very different properties. Ca(2+)-activated Cl- currents showed outward rectification, relatively slow activation kinetics and marked voltage dependence. These results are consistent with the existence of two different outwardly rectifying Cl- channels in rat parotid cells. 3. In conditions designed to eliminate the activation of these two Cl- currents, a third type of current was observed. This third current was activated in a time-dependent manner by hyperpolarized potentials and was about equally permeant to Cl-, I- and Br-. 4. The properties of the hyperpolarization-activated current were similar to those of the cloned ClC-2 channel. Polymerase chain reaction-based methods and ribonuclease protection analyses indicated the presence in parotid gland of mRNA homologous to ClC-2. 5. Individual parotid acinar cells expressed all three types of Cl- channels. Each type of channel may contribute to Cl- efflux in distinct stages of the secretion process depending on the intracellular [Ca2+], cell volume and membrane potential.