The anesthetic effect of lidocaine was evaluated in rats by determining the change in anesthetic requirement of cyclopropane MAC that was produced by blood concentrations of lidocaine in the clinically useful range. A linear reduction in anesthetic requirement was produced with concentrations up to 1 mug/ml. Further increases in lidocaine up to 5.5 mug/ml resulted in no further decrease in cyclopropane requirement. Lidocaine was found to contribute a maximum MAC fraction of 0.4.