Labeling of 2-mercaptopropionylglycine (2-MPG) with 99mTc, was studied and its chemical characteristics were examined. Further, biliary excretion behavior of this complex was comparatively estimated in mice, rats, and rabbits. 99mTc-2-MPG was rapidly excreted in large quantities into the bile in mice and rats: within 1 hr after injection, 51% of the injected dose was recovered from the bile in rats. On the other hand, the ligand exchange reaction between this complex and penicillamine indicates that a low hydrolyzed 99mTc species is coordinated with 2-MPG. These results suggest that a low hydrolyzed 99mTc is an effective feature in biliary excretion behavior of 99mTc compounds. Another interesting in vivo behavior of 99mTc-2-MPG is the difference observed in mice and rabbits: in mice, very high 99mTc activity is concentrated in the gallbladder and the clearance from tissues other than the gallbladder is rapid, whereas in rabbits, although a rapid and high excretion into the gallbladder is observed, a considerable high 99mTc activity is retained in the liver and the kidney. One reasons for this different in vivo behavior is the low stability of this complex at high dilution: a big animal has the large dilution volume which lead to higher decomposition estimated by the higher liver and kidney retention or the lower bile excretion. In conclusion, studies carried on 99mTc-2-MPG showed a good biliary excretion behavior but an in vivo unstableness in big animals.