Voltage-gated channels are not only essential to countless physiological functions in heart, but also serve as the molecular target of diverse anti-arrhythmic agents. With the recent cloning, sequencing, and expression of voltage-gated Na and K channels, the stage is now set to identify the key amino acid residues primarily involved in anti- arrhythmic action on voltage-gated Na and K channels. In particular, wild-type and mutant versions of both a voltage-gated Na channel derived from adult skeletal muscle (mu1, Trimmer et al., 1990), as well as a voltage-gated K channel derived from human heart (HK1, or Kv1.4, Tamkun et al., 1991) will be expressed in a mammalian cell line and probed by patch-clamp methods to detect functional changes in the effects of internally applied QX-314 (for mu1) and clofilium (for HK1), permanently charged versions of anti-arrhythmic agents. Mutations will be made according to recent work localizing the probable site of action of internally active anti-arrhythmic agents to the cytoplasmic vestibule of Na and K channels (Gingrich et al., 1993; Backx et al., 1992, Choi et al., 1993). Unmasking the arrangement of amino acids that are important for drug action would be a crucial prerequisite to unravelling the mechanism of anti-arrhythmic action at the molecular level. Such understanding would be invaluable in rational drug therapy and design.