The health effects of ambient PM 2.5 and its potential mechanisms have generated considerable interest. In vitro cell studies and ex vivo animal experiments may not accurately determine the characteristics of PM 2.5 particles. To better understand their detailed mechanisms, we performed an in vivo study using single photon emis-sion tomography (SPECT) imaging. To mimic the PM 2.5 particles, SiO2 nanoparticles modified by ethylene car-bonate or polyvinyl pyrrolidone were labeled with 131I. After administration via inhalation, in vivo SPECT imag-ing of the radiolabeled particles in sprague dawley rats was performed. It was found that radioactivity accumulated in the lungs and trachea 6 and 24 h after administration. In addition, significant radioactivity was observed in the abdomen, including the liver and kidneys. The results were also confirmed by ex vivo autoradiography. This study revealed that in vivo SPECT imaging could be an effective method for investigating the properties of PM 2.5 particles.