The efficacy of intravenously administered lidocaine 0.75 mg/kg and 1.5 mg/kg to protect against cardiovascular reactions associated with laryngoscopy and tracheal intubation was studied in two comparable groups of ten patients and compared with a similar control group of ten patients given only saline. Following laryngoscopy and tracheal intubation, the 1.5 mg/kg dose afforded complete protection against cardiac arrhythmias of all types. The smaller dose was ineffectual in this respect. While the larger dose caused borderline protection against hypertension and tachycardia, the smaller dose prevented only the rise in systolic blood pressure. Possible mechanisms to account for these observations are discussed. These include a direct myocardial depressant effect, a central stimulant effect, a peripheral vasodilating effect and finally an effect on synaptic transmission.