L,L-ethylenedicysteine (L,L-EC) can be labeled efficiently with 99mTc at pH 12 to obtain a highly pure and very stable tracer agent (99mTc-L,L-EC). The biological behavior of 99mTc-L,L-EC was studied in mice and a baboon. In mice, 99mTc-L,L-EC demonstrated a more rapid urinary excretion and less retention in the kidneys, the liver, the intestines, and the blood than did 99mTc-MAG3 at 10 and 60 min p.i. Urinary excretion decreased in probenecid pretreated mice, which indicates active tubular transport. In the baboon, the renograms for 99mTc-MAG3 and 99mTc-L,L-EC were comparable. Plasma-protein binding of 99mTc-L,L-EC was lower than that of 99mTc-MAG3 while its distribution volume and 1-hr plasma clearance were clearly higher. The promising results of the animal experiments suggest that 99mTc-L,L-ethylenedicysteine may be a useful alternative to 99mTc-MAG3 for renal function studies in humans.