To determine whether neuronal firing is affected by anesthetics, the behavior of neurons in the mesencephalic reticular formation in response to repetitive somatosensory stimulation at 2/sec was studied during wakefulness and during nitrous oxide, halothane, and thiopental anesthesia in the cat. With clinical doses of the three anesthetics there were simultaneous blockades of both initial facilitation (28 of 32) and subsequent habituation (31 of 32) of the evoked responses in a majority of the mesencephalic reticular formation neurons. This dishabituation phenomenon was most prominent with thiopental and least prominent with nitrous oxide. Thus, it is suggested that progressive inactivation of not only the facilitatory process but also the inhibitory process in the neuronal activities of the mesencephalic reticular formation is associated with the so-called “anesthetic state” produced by these three anesthetics.