Spontaneous oscillation of activity in olfactory system is commonly observed in various species and is thought to be important for odor-information processing. The procerebrum (PC) of a terrestrial slug Limax marginatus is the center for odor processing and learning. The PC shows an oscillatory local field potential (LFP) at about 0.7 Hz, which is synchronized over the entire PC and is modulated by odor stimuli1. However, little is known about how the PC codes odor information. In our previous study, we found that the PC dynamics possesses a long-term correlation, which is depended on innervations from tentacles. Here, we investigate how information is included in the oscillatory activity of PC under the administration of a conditioned odor. We used the Tone-Entropy (T-E) analysis to compare aversive-conditioning slugs with naïve slugs in the odor-evoked response. Neuronal firing rates are modulated through the two operations of excitatory and inhibitory synapses. Inter-spike intervals are considered to be made shorter or longer on a beat-by-beat basis by assumed physiological mediators: accelerator and inhibitor. The T-E analysis calculates two indexes: tone, for balance between two operations, and entropy, for total activity of both mediators2. After the administration of conditioned odor, the mean entropy value significantly increased in both naïve (3.3 to 3.8) and conditioned groups (3.3 to 4.0), whereas the mean tone value increased in conditioned group alone (naïve: -0.039 to -0.030, conditioned: -0.032 to 0.045). The results suggest that the balance of excitatory and inhibitory synapses in PC is altered with aversive conditioning.