Pulmonary study using hyperpolarized (HP) (129)Xe gas as an imaging medium must focus on dissolved-phase signals to make the most of the characteristic affinity of xenon for biological tissues, including blood. However, the spectral pattern of these signals differs between mice and other animals, including rats, canines, and humans. Dissolved-phase study has been reported only scarcely in mice, so spectral assignment has been an important subject for HP (129)Xe magnetic resonance (MR) spectroscopy (MRS) and MR imaging for its wider application. We performed MRS, including magnetization transfer experiments, and MR imaging studies to confirm the origin of dissolved-phase signals of mice ex vivo and in vivo and obtained evidence to assign dissolved-phase signals at 192 ppm for epicardial fat, 196 ppm for lung parenchyma, and 200 ppm for blood. These results were the first to show the possibility of fast exchange of xenon between plasma and red blood cells.