Lidocaine (L) levels were measured by gas chromatography (GC) and a new technique of direct measurement by enzyme immunoassay (EMIT) in 50 consecutive patients admitted to the Coronary Care Unit who received L. There was no significant difference between the levels by the two techniques. There was eradication of most ventricular arrhythmias in 78% of patients, partial effectiveness in 16%, and no improvement in 6%. Toxicity was mainly mild central nervous system effects, noted in 26 of the 50 patients. All six patients with serum L above 5.9 microgram/ml had signs of toxicity. EMIT was found to be accurate and much faster than the traditional GC method.