1. Paramyotonia congenita (PC) is a human hereditary disease caused by one or more amino acid substitutions in skeletal muscle sodium channels. Using macropatches, the effect of PC mutations R1448C and T1313M were compared with wild-type (WT) in Xenopus oocytes coinjected with both alpha- and beta-subunits of human skeletal muscle (SkM1) sodium channels. 2. Slow inactivation in either T1313M or R1448C was not different from WT. Fast inactivation in both PC mutants, however, was significantly altered. 3. Commonly used biophysical protocols (such as I-V curves, steady-state inactivation curves, and measurements of inactivation rates) did not uniformly indicate that hyperexcitability should result from T1313M or R1448C. In fact, the only alteration of fast inactivation common to T1313M and R1448C that predicted cellular hyperexcitability was slowed open-state inactivation, compared with WT. 4. To test whether this alteration was sufficient to cause the phenotypic hyperexcitability, we used a novel voltage command that simulated muscle membrane activity. With this protocol, we found that R1448C and T1313M were similar in that they maintained a significantly higher channel availability during high frequency activity, compared with WT.