The development and initial evaluation of a high-resolution single photon emission tomography (SPECT) based system to image the biodistribution of radiolabeled tracers such as Tc-99m and I-125 in unrestrained/un-anesthetized mice. An infrared (IR) based position tracking apparatus has been developed and integrated into a SPECT gantry. The tracking system is designed to measure the spatial position of a mouse's head at a rate of 10-15 frames per second with sub-millimeter accuracy. The high resolution, gamma imaging detectors are based on pixelated NaI(Tl) crystal scintillator arrays, arrays of compact position-sensitive photomultiplier tubes and novel readout circuitry for lower device cost while retaining high spatial resolution. Two SPECT gamma camera detector heads based on a 4 × 8 array of Hamamatsu R8520-C12 position sensitive photomultiplier tubes have been built and installed onto the gantry. The IR landmark-based pose measurement and tracking system is under development to provide animal position data during a SPECT scan. The animal position and orientation data acquired by the IR tracking system is used for motion correction during the tomographic image reconstruction.