Parasympathetic impairment post-myocardial infarction (MI) increases the likelihood of adrenergic induced arrhythmias. The mechanism underpinning the development of this phenotype is unknown, but may involve changes in oxidative stress post-MI. To investigate this we used a murine model 3 days post MI, sham surgery or naive single cage housing. Anaesthesia was induced with intramuscular ketamine (30mg kg^-1)/metatomidine (0.6mg kg^-1) and maintained with 2% isofluorane in oxygen; this was discontinued after repair of the thoractomy. On final skin closure metatomidine was reversed with atipamezole (2mg kg^-1; I.M.). Post-operative care included analgesics and subcutaneous isotonic dextrose/saline. All animals were humanely killed. The response to vagal nerve stimulation in isolated atrial preparations was increased at 3 days post-MI (naive n=10, -90 ± 12 bpm (mean ± S.E.M.) vs infarct n=11, -117 ± 14 bpm, p<0.05, 2-way ANOVA with Bonferroni post-hoc analysis). Non-specific NOS inhibition failed to suppress vagal response in the MI and sham groups. There were no significant differences in chronotropic response to carbamylcholine with or without NOS inhibition suggesting a presynaptic mechanism of altered NOS function. In the MI group there was an increase in right atrial superoxide response to NADPH and decreased cytoplasmic SOD expression. The failure of NOS inhibition to diminish vagal responsiveness in the MI and sham groups suggests that NOS is uncoupled. In the 3 day murine MI model vagal activity may be increased through a mechanism independent of the NO-cGMP pathway. Superoxide is a potential substrate that facilitates vagal neurotransmission in this model.