[en] Full text: The design, calibration, radiation dosimetry and preliminary performance evaluation of a prompt-gamma neutron activation analysis facility for in vivo body composition studies in small animals (i.e. rats or rabbits) are described. The system design was guided by Monte Carlo neutron and photon transport calculations performed using the MCNP-4C code. The facility utilizes a 555 GBq (15 Ci) Am-Be radionuclide neutron source positioned within a graphite collimator and appropriate shielding assembly. Prompt gamma rays produced by thermal neutron capture reactions within the animal are detected by a combination of a NaI(Tl) and a HPGe detectors positioned on either side of the sample, perpendicularly to the neutron beam. Small animal body nitrogen and hydrogen are determined by the NaI(Tl) detector by analysis of the 10.83 MeV and 2.22 MeV peaks, respectively, while calcium and chlorine are determined by the HPGe detector by analysis of the 6.42 MeV and 6.11 MeV peaks, respectively. Moreover, body potassium is determined independently by means of ⁴⁰K measurement at a modified whole body counter facility. Appropriate corrections for animal body size and shape are applied. Mixed neutron and gamma radiation dosimetry was performed using a tissue-equivalent proportional counter. The facility described is a simple tool enabling us to perform in vivo analysis of the major body compartments of protein, bone mass, extra-cellular and intra-cellular space. It will be used to perform serial nutritional and metabolic studies in sets of small experimental animals under controlled conditions for an ethically accepted radiation dose and without the need to kill the animal. (author)