Native myocardial voltage-gated sodium (Na V) channels function in macromolecular complexes comprising a pore-forming (α) subunit and multiple accessory proteins. Here, we investigated the impact of accessory Na V β1 and Na V β3 subunits on the functional effects of 2 well-known class Ib antiarrhythmics, lidocaine and ranolazine, on the predominant Na V channel α subunit, Na V 1.5, expressed in the mammalian heart. We showed that both drugs stabilized the activated conformation of the voltage sensor of domain-III (DIII-VSD) in Na V 1.5. In the presence of Na V β1, the effect of lidocaine on the DIII-VSD was enhanced, whereas the effect of ranolazine was abolished. Mutating the main class Ib drug-binding site, F1760, affected but did not abolish the modulation of drug block by Na V β1/β3. Recordings from adult mouse ventricular myocytes demonstrated that loss of Scn1b (Na V β1) differentially affected the potencies of lidocaine and ranolazine. In vivo experiments revealed distinct ECG responses to ip injection of ranolazine or lidocaine in WT and Scn1b-null animals, suggesting that Na V β1 modulated drug responses at the whole-heart level. In the human heart, we found that SCN1B transcript expression was 3 times higher in the atria than ventricles, differences that could, in combination with inherited or acquired cardiovascular disease, dramatically affect patient response to class Ib antiarrhythmic therapies.