Voltage-gated sodium channels, which initiate action potentials in mammalian brain neurons, are modulated functionally by cAMP-dependent protein kinase A (PKA), resulting in reduced sodium current amplitude. Comparing brain and muscle sodium channels, we show that only the brain channel is modulated by PKA. The brain sodium channel I-II linker is both necessary and sufficient for PKA modulation, as shown by exchanging the I-II linker regions of the two channels. PKA consensus sites in the brain channel I-II linker were eliminated by deletion and site-specific mutagenesis. The mutant channels demonstrated decreased levels of phosphorylation when metabolically labeled in oocytes with [gamma-32P]-ATP, and they did not respond with a reduction in current magnitude after PKA induction. Modulation of the brain channel by PKA phosphorylation was mimicked by adding fixed negative charges at the PKA consensus sites, suggesting that the decrease in current was a direct result of the negative charge at one or more of the PKA sites in the I-II linker.