Brain sodium channels are a complex of alpha (260 kDa), beta 1 (36 kDa), and beta 2 (33 kDa) subunits, alpha subunits are functional as voltage-gated sodium channels by themselves. When expressed in Xenopus oocytes, beta 1 subunits accelerate the time course of sodium channel activation and inactivation by shifting them to a fast gating mode, but alpha subunits expressed alone in mammalian cells activate and inactivate rapidly without co-expression of beta 1 subunits. In these experiments, we show that the Chinese hamster cell lines CHO and 1610 do not express endogenous beta 1 subunits as determined by Northern blotting, immunoblotting, and assay for beta 1 subunit function by expression of cellular mRNA in Xenopus oocytes. alpha subunits expressed alone in stable lines of these cells activate and inactivate rapidly. Co-expression of beta 1 subunits increases the level of sodium channels 2- to 4-fold as determined from saxitoxin binding, but does not affect the Kd for saxitoxin. Co-expression of beta 1 subunits also shifts the voltage dependence of sodium channel inactivation to more negative membrane potentials by 10 to 12 mV and shifts the voltage dependence of channel activation to more negative membrane potentials by 2 to 11 mV. These effects of beta 1 subunits on sodium channel function in mammalian cells may be physiologically important determinants of sodium channel function in vivo.