A transport mechanism of methylmercury to egg albumen was investigated in laying Japanese quail which had received a single intravenous injection of 203Hg-methylmercuric chloride. Whole body distribution of 203Hg was analyzed by means of autoradiography. It was notable that the uptake of 203Hg was higher in the oviduct than in any other tissue checked. Further, the 203Hg was detectable in egg albumen which had been synthesized and secreted in the magnumean portion of the oviduct. From the observations mentioned above, it is suggested that methylmercury in the vascular system was taken up cumulatively by the oviduct and deposited in egg albumen by the secretory way. On the tissue level of the benzene-extractable form of 203Hg compounds (RHg) quantitative analysis was made in the respective organs. RHg decreased from the blood and liver with the lapse of time. The renal level of RHg showed a gradual increase which was followed by a rapid decrease. In contrast to the tissue tested above, a magnumean uptake of RHg increased gradually and reached the highest level of the four tissue after 24 hours. The RHg content was analyzed in the egg shell, albumen and yolk. It was the largest in the albumen and second largest in the yolk, but negligible in the shell. In 7 eggs laid over a period of 8 days after the injection, the total RHg content was approximately 44% of the dose of the radiocompound administered. The largest amount of RHg was found in the albumen of an egg the third in the order of oviposition after the injection. The fecal and urinary content of 203Hg was approximately 23% of the dose during the same period of time. These quantitative data suggest that the major part of methylmercury in the body is transported into egg albumen via the secretory mechanism of the magnumean portion of the oviduct. Starch gel electrophoresis of the soluble cytoplasmic fraction of the magnumean homogenate and of egg albumen revealed that 203Hg was associated with protein bands which had migrated near the front. The protein bands were considered to be ovalbumin. It was confirmed from an examination in vitro that 203Hg-methylmercuric chloride bound to a purified hen's ovalbumin preparation. These results mentioned above suggest that methylmercury is readily bound to ovalbumin which was newly synthesized in large quantity in the magnumean portion of the oviduct.