The neurotoxins veratridine and aconitine are best known for their abilities to open action potential sodium channels in neuronal and muscle cells. These neurotoxins were tested for their abilities to influence the binding of 3H-QNB in mouse forebrain employing a glass-fiber filtration assay. Veratridine and aconitine produced approximately 95% and 77% inhibition of muscarinic receptor specific 3H-QNB binding with IC50 values of 11 and 20 microM, respectively. Further analysis revealed that both veratridine and aconitine were competitive inhibitors of 3H-QNB binding. Tetrodotoxin, an antagonist of the actions of veratridine and aconitine on sodium channels, at 10 microM inhibited the binding of 3H-QNB by 9% in the presence of 10 microM veratridine while having no effect when used in combination with aconitine. These data indicate that there may be a relationship between muscarinic receptors and action potential sodium channels.