Virtual Environments (VEs) have been used as mood induction procedures. In this context, it is necessary to have instruments to analyze the emotional state during VE exposure. Objective techniques such as EEG should be evaluated for this purpose. The aim in this work was to study the changes in the brain activity with a portable EEG device during a negative mood induction based on a VE. A virtual park was used to induce a negative mood (sadness) in ten participants. Changes in the brain activity of subjects were compared between two moments (before and after emotional induction). Obtained results were in accordance with previous scientific literature regarding frontal EEG asymmetry, which supports the possibility of using the portable EEG as a reliable instrument to measure emotions in VE.