Cisplatin, a platinum-based drug, is an important weapon against many types of cancer. It induces apoptosis by forming adducts with DNA, although many aspects of its mechanism of action remain to be clarified. Previously, we found a role for the volume-sensitive, outwardly-rectifying (VSOR) Cl^— channel in cisplatin-induced apoptosis. To investigate the possibility that cation channels also have a role in the cellular response to cisplatin, we examined the activity of cation channels in cisplatin-sensitive KB-3-1 (KB) epidermoid cancer cells by the whole-cell patch-clamp method. A cation channel in KB cells, activated by hypotonic stress, was identified as the Ca²⁺-activated, intermediate conductance K⁺ channel (IK1), based on its requirement for intracellular Ca²⁺, its blockage by the blockers clotrimazole and TRAM-34 and its suppression by a dominant negative construct. Activity of this channel was not observed in the KCP-4 cell line, a cisplatin-resistant cell line derived from KB cells, and its molecular expression, observed by semi-quantitative RT-PCR and immunostaining, appeared much reduced. Cell volume measurements confirmed a physiological role for the channel as a component of the volume regulatory machinery in KB cells. A possible role of the channel in cisplatin-induced apoptosis was investigated. It was found that clotrimazole and TRAM-34 inhibited a cell viability decrease, as well as a caspase-3/7 activity increase, caused by cisplatin treatment, while 1-EBIO, an activator of the channel, had the opposite effect. Thus, activity of IK1 appears to mediate, at least in part, the response of KB cells to cisplatin treatment.