The in vivo measurement of mercury in kidneys was investigated by detecting prompt gamma-ray emission after neutron absorption. A low-power mobile nuclear reactor provides the neutrons, and the kidney phantom is filled with HgCl2 solution. The detection limit of mercury in the left kidney for in vivo measurement is 58 ppm under 7200-s irradiation/collection. Possible ways of lowering the detection limit, such as thermalizing the neutrons coupled with modifying the detection system, are discussed.