Voltage-sensitive sodium channels are responsible for the initiation and propagation of the action potential and therefore are important for neuronal excitability. Complementary DNA clones encoding the β_1 subunit of the rat brain sodium channel were isolated by a combination of polymerase chain reaction and library screening techniques. The deduced primary structure indicates that the β_1 subunit is a 22,851-dalton protein that contains a single putative transmembrane domain and four potential extracellular N-linked glycosylation sites, consistent with biochemical data. Northern blot analysis reveals a 1400-nucleotide messenger RNA in rat brain, heart, skeletal muscle, and spinal cord. Coexpression of β_1 subunits with α subunits increases the size of the peak sodium current, accelerates its inactivation, and shifts the voltage dependence of inactivation to more negative membrane potentials. These results indicate that the β_1 subunit is crucial in the assembly, expression, and functional modulation of the heterotrimeric complex of the rat brain sodium channel.