The F1473S mutation of the adult human skeletal muscle Na+ channel causes paramyotonia congenita, a disease characterized by muscle stiffness sometimes followed by weakness in a cold environment. The symptoms are relieved by the local anaesthetic mexiletine. This mutation, which resides in the cytoplasmic S4-S5 loop in domain IV of the alpha-subunit, was studied by heterologous expression in HEK293 cells using standard patch-clamp techniques. Compared to wild-type (WT) channels, those with the F1473S mutation exhibit a two-fold slowing of fast inactivation, an increased persistent Na+ current, a +18-mV shift in steady-state inactivation and a fivefold acceleration of recovery from fast inactivation; slow inactivation was similar for both clones. Single-channel recordings for the F1473S mutation revealed a prolonged mean open time and an increased number of channel reopenings that increased further up on cooling. The pharmacological effects of mexiletine on cells expressing either WT, F1473S or G1306E channels were studied. G1306E is a myotonia-causing mutation located within the inactivation gate that displays similar but stronger inactivation defects than F1473S. The hyperpolarizing shift in steady-state inactivation induced by mexiletine was almost identical for all three clones. In contrast, this agent had a reduced effectiveness on the phasic (use-dependent) block of Na+ currents recorded from the mutants: the relative order of block was WT>F1473S>G1306E. We suggest that the relative effectiveness of mexiletine is associated with the degree of abnormal channel inactivation and that the relative binding affinity of mexiletine is not substantially different between the mutations or the WT.