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/s was studied during wakefulness and during N2O, halothane and thiopental anesthesia in the cat. With clinical i.v. doses of the 3 anesthetics there were simultaneous blockades of initial facilitation (28 of 32) and subsequent habituation (31 of 32) of the evoked responses in a majority of mesencephalic reticular formation neurons. This dishabituation phenomenon was most prominent with thiopental and least prominent with N2O. Progressive inactivation of the facilitatory process and the inhibitory process in neuronal activities of the mesencephalic reticular formation apparently is associated with the so-called anesthetic state produced by these 3 drugs.