The cardiac enkephalin, methionine-enkephalin-arginine-phenylalanine (MEAP) alters vagally induced bradycardia when introduced by microdialysis into the sinoatrial (SA) node. The responses to MEAP are bimodal; lower doses enhance bradycardia and higher doses suppress bradycardia. The opposing vagotonic and vagolytic effects are mediated respectively by -1 and -2-phenotypes of the same receptor. Stimulation of the -1-receptor reduced the subsequent -2-responses. Experiments were conducted to test the hypothesis that the -receptor interactions were mediated by the monosialosyl ganglioside, GM-1. When the mixed agonist, MEAP was evaluated after nodal GM-1 treatment, -1-mediated vagotonic responses were enhanced and -2-mediated vagolytic responses were reduced. Prior treatment with the -1 selective antagonist 7-benzylidenaltrexone (BNTX) failed to prevent attrition of the -2-vagolytic response or restore it when added afterward. Thus the GM-1-mediated attrition was not mediated by -1-receptors or increased competition from -1-mediated vagotonic responses. When GM-1 was omitted, deltorphin produced a similar but less robust loss in the vagolytic response. In contrast however to GM-1, the deltorphin mediated attrition was prevented by pretreatment with BNTX indicating that the decline in response after deltorphin alone was mediated by -1-receptors and that GM-1 effectively bypassed the receptor. Whether deltorphin has intrinsic -1 activity or causes the release of an endogenous -1 agonist is unclear. When both GM-1 and deltorphin were omitted, the subsequent vagolytic response was more intense. Thus GM-1, deltorphin, and time all interact to modify subsequent -2-mediated vagolytic responses. The data support the hypothesis that -1-receptor stimulation may reduce -2-vagolytic responses by stimulating the GM-1 synthesis.