Single photon emission computed tomography (SPECT) is an important technology for molecular imaging studies of small animals. In this arena, there is an increasing demand for high performance imaging systems that offer improved spatial resolution and detection efficiency. We have designed a multipinhole small animal imaging system based on position sensitive avalanche photodiode (PSAPD) detectors with the goal of submillimeter spatial resolution and high detection efficiency, which will allow us to minimize the radiation dose to the animal and to shorten the time needed for the imaging study. Our design will use 8 x 24 mm2 PSAPD detector modules coupled to thallium-doped cesium iodide [CsI(Tl)] scintillators, which can achieve an intrinsic spatial resolution of 0.5 mm at 140 keV. These detectors will be arranged in rings of 24 modules each; the animal is positioned in the center of the 9 stationary detector rings which capture projection data from the animal with a cylindrical tungsten multipinhole collimator. The animal is supported on a bed which can be rocked about the central axis to increase angular sampling of the object. In contrast to conventional SPECT pinhole systems, in our design each pinhole views only a portion of the object. However, the ensemble of projection data from all of the multipinhole detectors provide angular sampling that is sufficient to reconstruct tomographic data from the object. The performance of this multipinhole PSAPD imaging system was simulated using a ray tracing program that models the appropriate point spread functions and then was compared against the performance of a dual-headed pinhole SPECT system. The detection efficiency of both systems was simulated and projection data of a hot rod phantom were generated and reconstructed to assess spatial resolution. Appropriate Poisson noise was added to the data to simulate an acquisition time of 15 min and an activity of 18.5 MBq distributed in the phantom. Both sets of data were reconstructed with an ML-EM reconstruction algorithm. In addition, the imaging performance of both systems was evaluated with a uniformity phantom and a realistic digital mouse phantom. Simulations show that our proposed system produces a spatial resolution of 0.8 mm and an average detection efficiency of 630 cps/MBq. In contrast, simulations of the dual-headed pinhole SPECT system produce a spatial resolution of 1.1 mm and an average detection efficiency of 53 cps/MBq. These results suggest that our novel design will achieve high spatial resolution and will improve the detection efficiency by more than an order of magnitude compared to a dual-headed pinhole SPECT system. We expect that this system can perform SPECT with submillimeter spatial resolution, high throughput, and low radiation dose suitable for in vivo imaging of small animals.