A decrease in hepatic clearance and volume of distribution in patients with congestive heart failure frequently leads to toxicity when drugs such as lidocaine are administered. To determine the effect of adjusted dosing of lidocaine in patients with myocardial infarction, we studied 32 patients receiving lidocaine either by a conventional method (control group: 1–2 mg/kg bolus, then 1 to 4 mg/min) or by an adjusted regimen based on the presence or absence of heart failure [experimental group: 1–2 mg/kg bolus; then, class I (no heart failure), 35 to 88 μg/kg/min; class II (heart failure), 12 to 35 μg/kg/min]. Plasma lidocaine levels were determined at 2 and 5 h of the infusion by enzyme multiplied immunoassay technique (EMIT®) and gas liquid chromatography (GLC). Ten of 33 determinations in the control group were in the toxic range, i.e., > 6 μg/ml, and four others were subtherapeutic, i.e., < 2 μg/ml. In contrast, 13 of 16 determinations in the experimental group were in the therapeutic range, and none were in the toxic range. These data show that administration of lidocaine by a conventional method may produce diverse plasma levels that may occasionally be in the toxic range. Modified dosing based on cardiac status may result in optimal levels in most patients.