The interneuronal messenger nitric oxide (NO) may play a central role in the processing of olfactory information. Several circuit elements in the mammalian olfactory bulb contain NO synthase or its functional equivalent, NADPH diaphorase. The effects of NO on cellular excitability or circuit dynamics in the olfactory bulb are unknown, although NO effects on other rhythmic cells and circuits have been described. I have studied the role of NO in central olfactory processing using the procerebral (PC) lobe, the major central site of odour processing in terrestrial molluscs. As in the mammalian olfactory bulb during odour stimulation, the basic dynamics of electrical activity in the molluscan PC lobe is an oscillation. Here I report an obligatory role for NO in the oscillatory dynamics of the PC lobe of Limax maximus. Nitric oxide mediation of the olfactory oscillation may relate to the highly developed odour sensitivity and odour-learning ability of Limax.