The EEG is desynchronized during wakefulness and REM sleep. There are awake and REM sleep-related neurons in the brain stem. This study was carried out to investigate if the same neuron in the brain stem reticular formation may be responsible for EEG desynchronization during wakefulness and REM sleep. Single neuronal activity was recorded in chronically prepared freely moving normal cats and their activities were correlated with EEG desynchronization during spontaneous wakefulness, REM sleep, and during wakefulness induced by stimulation of the brain stem reticular formation. A majority of the neurons showed an increased firing associated with spontaneous EEG desynchronization during wakefulness and REM sleep, however, about 55% of them showed a similar behavior during stimulation-induced desynchronization. It was found that responses of a majority of the neurons during stimulation-induced desynchronization were similar to that of their firing rate during EEG desynchronization associated with spontaneous wakefulness irrespective of their behavior during REM sleep; the REM-ON neurons were not affected by the stimulation-induced desynchronization. A majority of the neurons which showed an increased firing during spontaneous and stimulation-induced EEG desynchronization received an excitatory input from the brain stem reticular formation. The results of this study suggest that although some neurons may be common, there is a strong possibility that the same neuron in the brain stem reticular formation is not involved in EEG desynchronization during wakefulness and REM sleep.