The rapid and transient generation of pulses of either cAMP or IP3 is considered the primary reaction in olfactory signal transduction. There is some recent evidence suggesting that another second messenger system, involving cGMP, may also play an important role in olfactory signalling. High doses of odorant elicit a delayed and sustained elevation of cGMP levels due to the operation of the nitric oxide (NO) synthase/guanylyl cyclase system. The interplay of NO and cGMP is supposed to trigger molecular mechanisms, including adaptation processes, which enable the olfactory neuroepithelium to cope with strong stimuli. Furthermore, a characteristic pattern of staining for NO synthase in the olfactory bulb suggests that the NO/cGMP system may also participate in neuronal processing of the sensory input.