Cholinergic neurotransmission is known to have important arousal/activating functions. The neurons responsible for those actions also release the atypical neuromodulator nitric oxide (NO), which has been shown in previous studies to be involved in the modulation of sleep/wake states. The present investigation, using an animal model (anesthetized cat) tests the hypothesis that NO cooperates with ACh in controlling rhythmic neuronal activity, which may play a role in sleep/wake transition. We have used extracellular singleunit recording of neurons in the dorsal thalamus and visual cortex with simultaneous iontophoretic application of drugs acting upon the NO system: the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine (L-NOArg), NO donors, and 8-bromo-cGMP (which mimics the action of NO). Local inhibition of NOS significantly reduced the activity of recorded cells in both thalamus and visual cortex. The opposite effect was achieved with NO donors application. In cortex, ejection of 8-bromo-cGMP or the NO donor diethylamine-nitric oxide (DEA-NO) increased cell firing. Furthermore, the rhythmic firing pattern present in these cortical neurons was disrupted. Taken together, these findings suggest that the NO system collaborates with cholinergic neurotransmission. This collaboration might be involved in the control of different patterns of electrogenic activity during various states of the sleep-wake cycle, via the ability of the NO system to modify rhythmic activity of neurons.