Two kinds of novel thymidine derivatives, N-thymidine-yl-N′-methyl-N′-N′′-[2-sulfanyl-(ethylamino)acetyl]-2-aminoethylsulfanyl-1-hexanamide}-ethanediamine (TMHEA) and N-thymidine-yl-N′-methyl-N′-N′′-[2-sulfanyl-(ethylamino)acetyl]-2-aminoethylsulfanyl-1-hexanamide}-hexanediamine (TMHHA) were prepared and successfully labeled with (99m)Tc in high labeling yields. The in vitro stability and in vivo