It has been an usual approach for the study of organic mercury metabolism to analyze the total mercury in tissues and excreta as a whole, owing to the difficulties in differentiating organic mercury from inorganic one. The Miller's method for the determination of phenylmercuric compounds, i.e. the extration by an organic solvent under very low pH following the mild alkaline digestion of organic matters, was found also to be appliable to ethylmercuric compounds by Nishimura and the author. Based on this fact, the short-term metabolism of ethylmercuric phosphate in rabbits and chronic intoxication in rats were investigated. When phenylmercuric phosphate was injected intramuscularly to rabbits, the peak in urinary excretion came on the second day and it decreased grandually until the sixth day, when no more organic mercury was detected in the urine. The rats were given water containing 5 ppm of ethylmercuric phosphate ad libitum for150 days, thereafter it was increased to 50 ppm for 23 days to induce poisoning symptoms in a shorter period. For the comparison, the inorganic mercury (mercury nitrate) was given to the positive control group in parallel. The animals were sacrificed at the end of the experiment and fractional determinations of mercury in the brain, liver and kindney were conducted. It was clearly demonstrated that the organs contained mercury in high concentration, of which the major portion was of organic nature. Hence the total mercury content in the organs amounted to three to seven times as high as that in the inorganic mercury group. However, even in the organic mercury group, some "apparent inorganic" mercury (inextractable mercury) was always found in organs over the usual range. It was uncertain for the present, whether it had come from the degredation of organic compounds to inorganic ones in the body or merely the fixation of organic mercury to some proteins occurred which was very resistant to the mild alkaline digestion and extraction procedures.