To understand the common computational principle of the olfactory coding and decoding mechanisms, we studied the peripheral olfactory system and higher-order olfactory center of the terrestrial slug Limax marginatus by physiological and histological techniques. We first examined the effects of electrical stimuli to the digits of tentacular ganglia on the frequency of electrical oscillations in the procerebral ( PC ) lobe both in the superior and inferior tentacles. Next, we selectively stained the output neurons that send their axons to the PC lobes to compare the superior and inferior tentacles histologically. Because these stained output neurons were considered to transmit the odor information including odor preference to the PC lobe, we also analyzed the odor-induced changes in the neural activity in the tentacular nerves. To obtain insight on the relationship between the tentacular nerve activity and the activity in the other olfactory structures from the peripheral to central organs, we analyzed the spontaneous activity in some slug olfactory pathways including the digits, tentacular ganglia, tentacular nerves, and PC lobe. Finally, we examined the distribution of nitric oxide ( NO )-generative cells and NO-responsive cells in the tentacles and PC lobes and found that the NO/cGMP pathways are involved in slug olfactory processing.