Skeletal and heart muscle excitability is based upon the pool of available sodium channels as determined by both fast and slow inactivation. Slow inactivation in hH1 sodium channels significantly differs from slow inactivation in hSkM1. The beta(1)-subunit modulates fast inactivation in human skeletal sodium channels (hSkM1) but has little effect on fast inactivation in human cardiac sodium channels (hH1). The role of the beta(1)-subunit in sodium channel slow inactivation is still unknown. We used the macropatch technique on Xenopus oocytes to study hSkM1 and hH1 slow inactivation with and without beta(1)-subunit coexpression. Our results indicate that the beta(1)-subunit is partly responsible for differences in steady-state slow inactivation between hSkM1 and hH1 channels. We also studied a sodium channel chimera, in which P-loops from each domain in hSkM1 sodium channels were replaced with corresponding regions from hH1. Our results show that these chimeras exhibit hH1-like properties of steady-state slow inactivation. These data suggest that P-loops are structural determinants of sodium channel slow inactivation, and that the beta(1)-subunit modulates slow inactivation in hSkM1 but not hH1. Changes in slow inactivation time constants in sodium channels coexpressed with the beta(1)-subunit indicate possible interactions among the beta(1)-subunit, P-loops, and the slow inactivation gate in sodium channels.