We used the patch-clamp technique to study the effect of arachidonic acid (AA) on basolateral 18 pS K channels in the principal cell of the cortical collecting duct (CCD) of the rat kidney. Application of AA inhibited the 18 pS K channels in a dose-dependent manner and 10 µM AA caused a maximal inhibition. The effect of AA on the 18 pS K channel was specific because application of 11,14,17-eicosatrienoic acid (EA) had no effect on channel activity. Also, the inhibitory effect of AA on the 18 pS K channels was abolished by blocking CYP epoxygenase with MS-PPOH but was not affected by inhibiting CYP -hydroxylase or cyclooxygenase. The notion that the inhibitory effect of AA was mediated by CYP epoxygenase-dependent metabolites was further supported by the observation that application of 100 nM 11,12-epoxyeicosatrienoic acid (EET) mimicked the effect of AA and inhibited the basolateral 18 pS K channels. In contrast, addition of either 5,6-, 8,9-, or 14,15-EET failed to inhibit the 18 pS K channels. Moreover, application of 11,12-EET was still able to inhibit the 18 pS K channels in the presence of MS-PPOH. This suggests that 11,12-EET is a mediator for the AA-induced inhibition of the 18 pS K channels. We conclude that AA inhibits basolateral 18 pS K channels by a CYP epoxygenase-dependent pathway and that 11,12-EET is a mediator for the effect of AA on basolateral K channels in the CCD.