Distribution of 35S-bis- (3, 5-dichloro-2-hydroxyphenyl) sulfoxide (35S -Bitin-S) in mice , rats and cockerels has been studied by means of whole body autoradiography and liquid scintillation counting. The radioactivity in male mice was marked in blood , liver, lung and connective tissues. The radioactivity in those tissues reached the highest level 6 hours after the administration, and thereafter decreased gradually . The radioactivity was not accumulated very much in the central nervous

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... , thymus, pancreas, spleen, brown fat, testis and endocrine glands. The 35S-Bitin-S or its metabolites passed the placental barrier and accumulated in certain tissues of the fetus . The concentration of radioactivity in fetal tissues was considerably lower than that in maternal tissues . The patterns of autoradiographic distribution of rats and cockerels were similar to those of mice . Approximately 70 % of the administered dose of 35S -Bitin-S was absorbed from the ligated loop of rat's intestine in 4 hours . The concentration of 35S-Bitin-S in the tissues of rat after oral administration was the highest in plasma , relatively high in the liver and lung, and the lowest in the brain . It was found that the high concentration in the plasma lasted for a long time. The amount of 35S excreted in urine of rat in 6 day period after the administration was only 6 % of the administered radioactivity , whereas that in faeces in the same period was as high as 65 %Introduction Hamada1) reported that bis-(3 , 5-dichloro-2hydroxyphenyl) sulfoxide (Bitin-S) has a stronger anthelmintic activity than bis-(3, 5dichloro-2-hydroxyphenyl) sulfide (bithionol) . The studies on the metabolic fate of 35S-Bitin-S were described in the previous paper2) . The main metabolite in bile was glucuronide of bithionol whereas metabolites in urine were sulfonic acid, inorganic sulfate, bithionol and its glucuronide. The present study was undertaken to know the distribution and excretion of 35S-Bitin-S in rats, pregnant mice and cockerels by the use of the whole body autoradio-graphy3) and liquid scintillation counting. Experimental (1) Radioactive compounds 35S-Bitin-S was synthesized from 35S -thiony 1chloride according to the previous paper2). The specific activity of the compound was 10 aCi/ mg and its radiochemical purity shown by paper chromatography was higher than 97%. The labelled Bitin-S is illustrated by the following formula: (2) Whole body autoradiographic technique 35S-Bitin-S was suspended in 0 .5% CMC (carboxy methyl cellulose) and was orally given to the animals (20 mg/kg in dose). In the first series 35S-Bitin-S was administered to the ddY