To explore neural mechanisms how olfactory information is processed in the brain and finally converted into behavior, it would be useful to have isolated whole brains that include both olfactory organs and motor output. In the present study, we identified an in vitro index of odor-evoked behavior in the terrestrial mollusk Limax and also studied the modulation of this in vitro index of the behavior. We determined that shortening of the mantle muscles is one of the withdrawal responses selectively induced by aversive odors and that the shortening is mediated by a pair of parietal nerves. We also identified a motoneuron (named the posterior visceral neuron, p-VN) that projects to the parietal nerve and innervates the mantle muscles. When we applied various odors to the nose in these isolated molluscan brains, only aversive odors induced discharges in the p-VN. These results indicate that p-VN discharges can serve as an in vitro index of odor-induced aversive behavior. We also identified a novel serotonergic neuron (named the posterior cerebral serotonergic cell, p-CSC). Discharges in the p-CSC released serotonin to the tentacle ganglion (TG); serotonin in the TG then inhibited odor-induced discharges in the p-VN, the in vitro index of aversive behavior. These results suggest that the serotonergic system is involved in the regulation of approach and avoidance behavior in Limax.