Nursing mothers are occasionally treated with intravenous lidocaine for ventricular dysrhythmias. There have been no reports on the excretion of lidocaine into breast milk. This case documents the excretion of lidocaine into breast milk in small amounts and shows the validity of the TDx methodology used in the whole-milk lidocaine assay. We observed breast-milk concentrations of lidocaine at 40 percent of the serum levels. Clinical practitioners should be aware the lidocaine is excreted into breast milk in small amounts and the mother could probably continue to safely breast-feed her child while on parenteral lidocaine. Any adverse reactions in the nursing infant would probably be limited to an idiosyncratic or allergic reaction.