In mammalian cells, little is known about the initial events whose ultimate consequence is mutagenesis or DNA repair. The role the plasma membrane may play as an initiator of such a pathway is not understood. We show, for the first time, that membrane voltage-dependent potassium (K+) currents, activated by ionizing radiation (Kuo et al., 1993), play a significant role in radiation mutagenesis. Specifically, we show that the frequency of mutation at the HGPRT locus is increased as expected to 37.6 +/- 4.0 mutations per 100,000 survivors by 800 cGy of ionizing radiation from a spontaneous frequency of 1.5 +/- 1.5. This increase, however, is abolished if either K+ channel blocker, CsCl or BaCl2, is present for 2 h following irradiation of the cells. RbCl, chemically similar to CsCl but known not to block K+ channels, is ineffective in reducing the mutation frequency. Treatment of cells with CsCl or BaCl2 had no effect on radiation-induced cell killing.