To determine the behavior of transthyretin (TTR) in blood circulation, TTR purified from normal subjects' plasma was injected to rats, and blood and urine were collected time dependently. Although TTR in plasma was proven to be a predominantly cysteine (Cys) conjugated form by electrospray ionization mass spectrometry (ESI-MS) analysis, it was gradually converted into free, 32 Da (dihydroxylation), 80 Da (phosphorylation), and 306 Da (glutathionylation), increased forms in molecular weight of TTR. The plasma levels of TTR were decreased in a time-dependent manner with the half life of 72.4 min. No secretion of TTR into the urine was observed by ESI-MS. In conclusion, this method can be simply performed without loading a radioactive molecule to the targeted protein. It offers a possibility to determine natural protein behaviors in the blood stream.