To characterize the volume-activated Cl(-) current in bovine pigmented ciliary epithelial (PCE) cells, the whole-cell patch clamp technique was employed. Exposure to a hypotonic solution induced a volume-activated Cl(-) current. The activation of the current depended on intracellular ATP. The current showed a characteristic outward rectification. There was no, or negligible, time-dependent inactivation of the current. The current-voltage relationship showed that the reversal potential of the hypotonic-activated current ( 6.3 0.5 mV) was close to the calculated equilibrium potential for Cl(-) (ECl=0 mV). Extracellular ATP inhibited both outward and inward currents, but the inhibition of outward currents was larger than that of inward currents (92% vs 74%, P<0.01). The current was also blocked by extracellular application of tamoxifen, a Cl( ) channel inhibitor, the effects of which were almost equal for both outward and inward currents (85% vs 87%, P>0.05). The properties of this current are quite similar to those of the volume-activated Cl(-) current associated with P-glycoprotein in other cell types, suggesting that P-glycoprotein may be involved in the activation of the volume-activated Cl(-) current in PCE cells.