Abstract Brain concentration of mercury as low as 100 ppm were determined by detecting the 368 keV prompt γ-radiation emitted during the 199Hg( n,γ) 200Hg reaction at doses of 102 mrem. Phantom studies demonstrated that the assay was accurate to within ±3,2%. A 60 cm 3 Ge(Li) detector of 2.14 keV resolution at the 60Co line was used to view the parietal region of the skull. Spatial consideration is given to the dose measurements. An approach to achieve minimum shielding weight and greatest reduction in background is described. A metallic artificial mole is used to control variation in head position.