Upon exposure to ultraviolet (UV) radiation, osmotic changes or the presence of reactive oxygen species (ROS) c-Jun N-terminal kinases (JNKs) are rapidly activated. Extensive studies have elucidated molecular components that mediate the activation of JNKs. However, it remains unclear whether activation of JNKs by various stress signals involves different pathways. Here we show that K+ channel activity is involved in mediating apoptosis induced by UV but not by H2O2 in myelocytic leukemic ML-1 cells. Specifically, JNKs were rapidly phosphorylated upon treatment of ML-1 cells with UV and H2O2. UV-induced, but not H2O2-induced, JNK-1 phosphorylation was inhibited by pretreatment with 4-aminopyridine (4-AP), a K+ channel blocker. 4-AP also blocked UV-induced increase in JNK activity as well as p38 phosphorylation. Immunofluorescent microscopy revealed that phosphorylated JNKs were concentrated at centrosomes in ML-1 cells and that these proteins underwent rapid subcellular translocation upon UV treatment. Consistently, the subcellular translocation of JNKs induced by UV was largely blocked by 4-AP. Furthermore, UV-induced JNK activation was blocked by NEM, a sulfhydryl alkylating agent also affecting K+ current. Both UV- and H2O2-induced JNK activities were inhibited by glutathione, suggesting that the redox status does play an important role in the activation of JNKs. Taken together, our findings suggest that JNK activation by UV and H2O2 is mediated by distinct yet overlapping pathways and that K+ channel activity and redox status are differentially required for UV- and H2O2-induced activation of JNKs.