Dietary polyunsaturated fatty acids (PUFAs) prevent ischemia-induced fatal cardiac arrhythmias in animals and probably in humans. This action results from inhibition of ion currents for Na+, Ca2+, and possibly other ions. To extend understanding of this protection we are seeking a possible binding site for the PUFAs on the alpha -subunit of the human cardiac Na+ channel, hH1(alpha), transiently expressed in HEK293t cells. Three mutated single amino acid substitutions with lysine were made in the alpha -subunit at Domain 4-Segment 6 (D4-S6) for F1760, Y1767 and at D1-S6 for N406. These are in the putative sites of binding of local anesthetics and batrachotoxin, respectively. The mutants F1760K, Y1767K, and N406K, separately and to different extents, affected the current density, the steady-state inactivation potential, accelerated inactivation, delayed recovery from inactivation, and affected voltage-dependent block, but did not affect activation of the hH1(alpha). It is essential to learn that single point mutations in D1-S6 and D4-S6 alone significantly modify the kinetics of human cardiac hH1(alpha) Na+ currents. The effects of PUFAs on these mutant channels will be the subject of subsequent reports.