Inducible nitric oxide synthase (iNOS) is a molecule of great interest, given the numerous biological activities of nitric oxide and the documented expression of iNOS in several CNS pathologies. There also appears to be species-dependent regulation of iNOS expression as well as CNS-specific regulation. In this study, we have examined cultures of cytokineactivated primary human astrocytes as a model system with which to study the mechanisms of iNOS regulation in human CNS. As one of the major functions of astrocytes is spatial buffering of K ϩ ion, we examined the effect of high extracellular KCl on astrocyte iNOS expression. The results demonstrate that KCl at 25-75 mM potently inhibits astrocyte nitrite production stimulated by interleukin-1 (IL-1)/interferon-␥ (IFN␥). In addition, several potassium channel inhibitors such as CsCl, tetraethylammonium, and 4-aminopyridine as well as nigericin inhibited astrocyte iNOS expression induced by IL-1/IFN␥. These results demonstrate a novel role for astrocyte potassium channel activity in modulation of astrocyte function. They further suggest neural-specific mechanisms for glial iNOS regulation.