Absorption and behaviors of the phosphate and benzoyl groups in S-benzoylthiamine O-monophosphate (BTMP) after the oral administration have been investigated in comparison with those of the thiamine moiety, using ^<32>P, ^3H (benzoyl)-labelled BTMP. After infusion of ^<32>P-BT MP in dog ligated intestinal loop, ^<32>P was found to be absorbed in the blood very rapidly, independent from the thiamine moiety. Blood ^<32>P, ^3H and thiamine concentration of rats orally given the double-labelled BTMP showed that, i) ^<32>P, after a fast rise and a decline, kept the highest level for over 24 hours, being incorporated in red cells, ii) ^3H level, after a fast rise, declined most rapidly, being almost negligible after 2 hours, and iii) thiamine showed an intermediate features of these two. Urinary excretion in 24 hours was ^3H (86%)>thiamine(38%)>^<32>P (12%), indicating that the phosphate group remained in the body for the longest period, while the benzoyl group for the shortest to be excreted in urine, possibly, as hippuric acid. Whole body autoradiography of ^<32>P-BTMP in mice showed that the highest activity was concentrated in bone and to a less extent in liver, the former being kept for over 120 hours. There was found no significant difference in the absorption and behaviors of radio-phosphorus between the administration of ^<32>P-BTMP and ^<32>P-inorganic phosphate. These results confirmed the mechanism of absorption of BTMP that after dephosphorylation of BTMP at the mucosal surface of intestine, S-benzoylthiamine is absorbed in the blood.