The organ distribution and biological half-time of methylmercury were examined in male mice of 3 inbred strains, BALB/c, C3H/HeN and C57BL/6N, and a random-bred strain, CD-1 (ICR), at 6 weeks after birth. Methylmercury chloride was administered intraperitoneally at the dose of 1 mg per kilogram of body weight. At various time intervals after methylmercury administration 9 to 12 mice of each strain were killed and the total mercury concentrations in the blood, brain, liver and kidneys were determined with cold-vapor atomic absorption spectrophotometry. A significant strain difference was found in the mercury concentrations in the organs, especially in the blood. BALB/c and C3H had twice as high blood mercury levels than C57BL and ICR. The brain/blood, liver/blood and kidney/blood ratios of mercury concentration appeared to be hereditarily stable characteristics. The biological half-time of mercury in the organs was longest in C57BL (except in the case of the kidneys) and shortest in ICR. Some biological features such as relative organ weight and body weight were considered to affect the chemobiokinetics of methylmercury in mice.