Thirty ppm or 3 ppm of mercuric chloride of protein bound form were mixed with commercial pellet diet and given to rats ad libitum for 4 months. Fifteen mg/kg or 1.5 mg/kg of methylcobalamin (CH_3-B_<12>) was orally administered daily throughout this period. Total mercury and methyl mercury levels in tissues, especially total mercury levels, increased in parallel with the mercury content in food and with the duration of feeding, but these increases were not related to the administration of CH_3-B_<12>. Furthermore, general toxicological findings were not deteriorated by CH_3-B_<12> but the toxic symptoms induced by inorganic mercury were found rather improved by the administration of CH_3-B_<12>. It is concluded from these results that biological methylation of inorganic mercury in vivo is not accelerated by oral administration of CH_3-B_<12>.