The microinjection of cholinergic drugs into the pontine reticular formation elicits active sleep-like states that are comprised of the principal physiological patterns of activity that characterize naturally-occurring active sleep, i.e., EEG desynchronization, PGO waves, rapid eye movements and atonia. We have reported that other behavioral states arise even when cholinergic drugs are injected into the exact same reticular location. The present study was conducted to explore the basis for the differences in the drug effect. A combination of acetylcholine and neostigmine was injected by microiontophoresis into the dorsal region of the nucleus pontis oralis in four chronic, unanesthetized cats. The states that were induced by cholinergic drug injection depended on the state of the animal at the time of the injection. When the animal was awake, cholinergic injections resulted in a waking-dissociated state, which was characterized by EEG desynchronization and muscle atonia in a cat that appeared to be awake and was able to track objects in its visual field. If the cat was in quiet sleep at the time of the injection, an active sleep-like state followed that was indistinguishable from naturally-occurring active sleep; on a few occasions following cholinergic injections during quiet sleep there was a quiet sleep-dissociated state, which was characterized by PGO waves and muscle atonia in the cat that by other indices appeared to be in quiet sleep. The results of this study indicate that the state of the animal at the time of drug injection is a critical variable that influences the responses which are induced by cholinergic stimulation of the pontine reticular formation.