PURPOSE To characterize the outward chloride currents (Cl(OR)) in single acinar cells isolated from the rabbit superior lacrimal gland (RSLG) to investigate the hypothesis that Cl(OR) may have a role in regulating the volume of RSLG acini. METHODS Cl(OR) was characterized by using patch-clamp electrophysiology. Confocal microscopy was used to measure intracellular calcium concentration ([Ca(2+)](i)) and cell volume. Cell volume was altered by superfusing the cells with a hyposmotic solution. RESULTS The Cl(OR) current contributed 33% of total membrane conductance. With normal osmotic conditions, the Cl(OR) current was activated by [Ca(2+)](i) with an EC(50) of 10(-8) M. A decrease in intracellular pH from 7.4 to 6.8 totally inhibited Cl(OR) current activity. Continuous superfusion of hyposmotic solution caused (1) an increase in cell volume that peaked within 4 minutes and gradually returned to baseline levels after 12 minutes, (2) an increase in [Ca(2+)](i) that peaked between 6 and 8 minutes and gradually returned to baseline levels after 15 minutes, and (3) an increase the Cl(OR) current that peaked within 6 minutes after commencement of perfusion and quickly returned to baseline levels. CONCLUSIONS The Cl(OR) current appears to be triggered by an increase in cell volume and then deactivates within the period of raised [Ca(2+)](i) during hyposmotic stress, suggesting that Cl(OR) may be an initiating event for volume homeostasis. This effect would be important during RSLG tear secretion, which usually involves cell volume changes and is accompanied by intracellular pH changes in the presence of the raised [Ca(2+)](i) to support secretion.