The differential effects of anesthetics on unitary behavior of the cat's mesencephalic reticular formation in response to repetitive electrical stimulation of the skin were studied by means of longterm microelectrode recording and computer analysis. In most units the anesthetics decreased the total number of firings in response to repetitive electrical stimulation of the skin. However, the ratio of evoked to spontaneous activity varied from unit to unit. Generally, the ratio was highest with sodium thiopental, and lowest during diethyl ether anesthesia. Evoked activity was easily distinguished from background activity during sodium thiopental and halothane anesthesia, but hardly distinguishable during diethyl ether anesthesia. The poststimulus times of short-latency discharges were hardly affected by the anesthetics in most units until the units ceased to respond to stimulation. In contrast, long-latency discharges were easily affected by anesthetics.