The incorporation and distribution of tritium were examined in rats exposed to tritiated rice or tritiated soybean by single ingestion or continuous feeding. Results were compared with those for tritiated wheat and tritiated water in a previous study done under the same experimental conditions. All the tritiated crops examined were more efficiently incorporated into rat tissues than was tritiated water, the extent of incorporation depending on the kind of crop. The differences in incorporation were clear in organically bound tritium determined as tritium in dried tissue. The respective concentrations of organically bound tritium after a single ingestion of tritiated rice, tritiated wheat or tritiated soybean were about 10-20, 20-30 and 25-60 times higher than after the ingestion of tritiated water. After continuous feeding for 22 days with tritiated rice, tritiated wheat or tritiated soybean, the respective concentrations of organically bound tritium were 5-8, 6-11 and 10-25 times the values after continuous ingestion of tritiated water. At the end of continuous ingestion, the radiation dose rates to almost of the tissues from all three tritiated crops were estimated to be 2-3 times that for tritiated water.

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Polished rice samples harvested in 1985 were collected from 25 prefectures throughout Japan. Concentrations of both tissue free water tritium (TFWT) and organically bound tritium (OBT) were determined. Nearly uniform distribution of the TFWT and OBT concentrations was observed in Japan, taking into account a relatively large counting error. The average values for all Japanese polished rice were 2.69 +/- 0.74 Bq/L for the TFWT concentration, 2.01 +/- 0.78 Bq/L for the OBT concentration and 0.83 +/- 0.32 for the specific activity ratio. The TFWT concentration was within the range of tritium concentration of the land water reported by other researchers. The specific activity ratio below unity also confirmed by analyses of the 4 Chinese polished rice samples suggests that OBT in the rice is not equilibrated with TFWT or HTO in the environment because of isotope discrimination.

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