The effects of various plasma concentrations of lidocaine on nitrous oxide anesthesia in man and halothane requirements in the dog were studied. The response to incision of the skin was observed in 20 patients who were anesthetized with nitrous oxide, 70% inspired, and oxygen, 30%, plus various plasma levels of lidocaine. In addition, changes in the MAC of halothane in dogs were observed at various levels of lidocaine. In both circumstances lidocaine concentrations of 3 to 6 microgram/ml decreased anesthetic requirements approximately 10 to 28%. At clinically common concentrations of lidocaine, significant decreases in anesthetic requirements should be anticipated.