To obtain further information about the neuronal mechanisms involved in the state of anesthesia, long-term microelectrode recording of activity in the mesencephalic reticular formation in the cat was undertaken. Results were subjected to computer analysis. In most units in the mesencephalic reticular formation (32 of 37 studied) spontaneous firing rates decreased as anesthesia deepened. However, the susceptibility of the spontaneous activity of the units to depression by anesthetic agents varied widely from unit to unit. Two types of changes in spontaneous firing patterns were produced by anesthetics: the “grouping” type and the “tonic” type. The “grouping” pattern was seen more often during sodium thiopental and halothane anesthesia, whereas the “tonic” type was more characteristic of nitrous oxide and diethyl ether anesthesia. In two of 37 units, synchronization between mesencephalic reticular formation unit firings and sharp positive waves of the corticogram was observed during the deep stage of sodium thiopental anesthesia.