In this paper we present a fully self-contained imaging instrument (30 mm overall length) that is capable of recording high speed and detect relatively small fluorescent signals (0.1% ΔF/F) from brain tissues potentially containing genetically-encoded sensors or dyes. This device potentially enables the study of neuronal activity in awake and mobile animals during natural behaviors without the stress and suppression of anesthesia and restraint. The device is a fully self-contained illumination system, wide field fluorescence microscope ( ~ 4.8mm² FOV-25 um lateral resolution-1.8 × magnification-0.39 NA) and CMOS image sensor (32 × 32). The total weight of the system is 10 g and is capable of imaging up to 900 fps. We present voltage dye RH1692 experiments using the system to study the somatosensory cortex of mice during whisker movements using an air puff.