Spontaneous firing of midbrain reticular formation (MRF) neurons was recorded extracellularly in chronically implanted, behaving cats during steady and transitional states of the sleep-waking cycle. Physiological identification of receiver and/or projection MRF neurons was achieved through orthodromically elicited discharges. Discharge rates of MRF neurons were more than double in waking (W) and active sleep (D) without phasic motor events, as compared to synchronized sleep (S). During behavioral states associated with EEG activation, the increased firing was essentially due to cells exhibiting high discharge rates, located at relatively ventral levels of the midbrain core. MRF neurons with identified rostrally projecting axons were more active during W and D states; their discharge rates were significantly higher than those of caudally projecting cells. The discharge patterns of MRF neurons increasing their firing rates from S to W and D were of the tonic type. First-order analyses showed a negligible proportion of both very short and long interspike intervals in all states, large interval density around the mode especially in W and D, and the smallest variation coefficients in W. Rhythmic firing with a period near the modal interval was detected during W by autocorrelations. The increase in firing rate of MRF neurons from S to W or D took place before overt EEG desynchronization and behavioral manifestations that define stable W or D states. In our sample a statistically significant increase in discharge rate was found about 15 s before the end of S sleep epochs that developed into awakening. The differences between discharge features of MRF neurons during waking and sleep states and those of neurons in other brainstem reticular fields are emphasized. Taken together, these data support, at a cellular level, Moruzzi and Magoun's concept of a rostral reticular substrate that gives rise to impulses leading to tonic activation of the thalamocortical systems.