Rhythmic electrical activity in the central nervous system is involved in cognitive functions in many animals and is also found in the olfactory centers of terrestrial gastropods, including slugs and snails. The procerebrum, the secondary olfactory center of the terrestrial slug (Limax valentianus), forms a unique layered structure that generates spontaneous, synchronized oscillatory activity all the time to process odor-related information. Previous studies have shown that synchronous oscillatory networks are spontaneously reorganized as neurites elongate in dispersed cell culture of procerebrum neurons, and that activation of the procerebrum neuronal network via cholinergic synapses is important for the generation of synchronous oscillatory activity. When differences between synchronous and asynchronous network were examined in cultured procerebrum neuron (7-21 days), lower excitability and a higher sensibility to cholinergic activation of procerebrum neurons were observed in synchronous networks. Muscarinic receptor agonist, pilocarpine, did not induce synchronous oscillation and did not occlude the excitatory effects of acetylcholinesterase inhibitors (physostigmine, neostigmine or donepezil). It is suggested that: (1) in vitro synchronous oscillation was induced by activation of cholinergic synaptic transmission via nicotinic acetylcholine receptors; (2) synchronous networks were characterized by increased cholinergic sensitivity together with the lateral inhibition of excessive activity in resting states. Comparison of morphological properties between synchronous and asynchronous oscillatory networks were also discussed. In order to elucidate the mechanism and the role of synchronous oscillations, we are investigating the process of formation of synchronous oscillatory networks in cultured procerebrum neurons in a bottom-up manner.