The behavior of animals is flexible and can be changed dramatically in response to the environmental stimuli. However, the neural basis of how environmental stimuli elicit an appropriate behavior is not clearly understood. We focused on the odor preference behaviors in Drosophila larvae as a model system and established a quantitative assay system for the larval odor preference behavior. Using this assay system, we found that the third instar larvae dramatically change their odor preference during the late larval stages. For example, early third instar larvae (72-96 hours after egg laying) are strongly attracted by 1-propanol, whereas wandering larvae (96-120 hours after egg laying) show less interest to the same odor. This suggests that neural circuits that determine the odor preference might be modified during 24 hours prior to metamorphosis. To elucidate the neural circuits underlying the preference change, we developed a preparation to image odor-evoked calcium signal in larval brain. Now identification of the neurons involved in the odor preference changes is ongoing. We would like to discuss how the brain makes decision and changes the decision against the same sensory information.