Arterial plasma lidocaine seizure thresholds were measured in rhesus monkeys, with and without gallamine triethiodide immobilization. Electrical brain activity was recorded by means of chronically-implanted electrodes placed in various cortical and subcortical depths. Immobilized animals had artificial respiration. Lidocaine was administered intravenously (4 mg/kg/ min) until electrical seizure activity began. In 11 experiments on five animals, mean ± standard deviation plasma lidocaine levels at which seizures occurred were 25.5μg/ml±3.0μg/ml in the control group, and 36.2μg/ml± 4.5μg/ml in the group treated with gallamine (P<.001). The 42% increase in seizure threshold during muscle relaxant immobilization indicates that gallamine possesses a significant effect on the sensitivity of the central nervous system to lidocaine. These findings are important both in consideration of drug interactions and in the use of muscle relaxant agents during physiologic experimentation.