The blood concentrations of lidocaine and its active monodeethylated metabolite, monoethylglycinexylidide (MEGX), were measured in 31 patients who were receiving infusions of lidocaine intravenously. In 3 patients who were studied intensively, the elimination half-life of MEGX was 120 min, which was similar to the elimination half-life of lidocaine (139 min). An additional 3 patients demonstrated a higher ratio of the concentration in blood of MEGX to lidocaine, and the MEGX may have contributed, in 1 patient, to the central nervous system toxicity that occurred during the infusion. Elevated concentrations of MEGX in blood were associated with congestive heart failure (r = 0.5, p = 0.004). Our data suggest that the elimination of MEGX may be decreased in patients with depressed cardiac output and sympathomimetic compensation.