We present for the first time in vivo imaging of rat brain using microwave-induced thermoacoustic tomography (TAT). The in vivo imaging of rat brain was realized through an unconventional delivery of microwave energy from the front of rat brain (while the transducer was scanned along coronal plane of the animal brain), which maximized the microwave penetration into the brain. In addition, we found that the imaging contrast was highly dependent on the direction of the electric field polarization (EFP) and that more tissue structures/compositions could be revealed when both X- and Y -EFPs were used for TAT. The in vivo TAT images of rat brain obtained were compared with the 3.0 T MRI images and histological photographs, and numerous important brain anatomical structures were identified. An example of our TAT approach for imaging a foreign object embedded in a rat brain was also demonstrated. This study suggests that TAT has a great potential to be used in neuroscience studies and in noninvasive imaging of brain disorders.