Abstract: The PAT molecule consists of Sb and tartaric acid components in equimolar amounts.If the equimolarity no longer exists after metabolism, it can be inferred that a cleavage of PAT must have taken place. From the view point of the chemistry of complex compounds, an in vivo cleavage of PAT molecules is quite a possibility as a result of the new equilibrium established between the ligands and centric ions in the serum. This paper deals with the quantitative relationship between the Sb and tartaric acid components of PAT during excretion into urine and bile, and during their influx into and efflux from the blood cells. Our results all showed that the molar concentration of Sb was not equal to that of tartaric acid, indicating the occurrence of an in vivo cleavage of PAT. After a single dose of 30 mg of PAT was administered intravenously to rabbits, tartaric acid elimination in urine was almost complete in about 6 hours, while the Sb excretion was so low that it amounted to only 18% of the total dose administered. On the other hand, the biliary excretion of tartaric acid was very low during the first hour after PAT administration, meanwhile as much as 29% of the administered Sb appeared in the bile. In the experiments of the influx into the blood cells, dog's blood was incubated in vitro with PAT. The amount of Sb remaining in the serum decreased to about half of the original level at one hour after the addition of PAT, and the Sb contents of the blood cells rose correspondingly, but the quantity of tartaric acid in the serum remained nearly unchanged. In the experiments of the efflux from the blood cells, serum was removed after dog's blood was incubated with PAT for 1(1/2) hours, then Locke solution was added instead of serum. It was found that the ratio of Sb and tartaric acid molarities in Locke solution was 5.7±2.5 at the end of the second 1(1/2) hour period. The fact that the quantitative changes of Sb and tartaric acid did not run parallel to each other during the above processes, revealed that the cleavage of PAT molecule could also take place in vitro and suggested that the blood might be one of the important tissues in which the in vivo cleavage occurred.