A new ligand, an N-p-iodophenethyl diaminodithiol (DADT-IPE), an anlog of N-isopropyl-p-iodoamphetamine (IMP), was synthesized and subsequently complexed with 99mTc, using stannous chloride as a reducing agent. Two complexes (a and b) were separated from 99mTc-DADT-IPE by high performance liquid chromatography (HPLC). Competitive inhibition studies showed that the IC50 value of DADT-IPE (70 microM) was similar to that of IMP (49 microM). Biodistribution studies of one of the complexes [99mTc-DADT-IPE(a)] in rats showed that 0.65% of the injected dose of the tracer remained in the brain at 5 min after intravenous injection, with 0.53% of the dose remaining in the brain at 60 min post-injection, whereas the corresponding values for the other complex [99mTc-DADT-IPE(b)] were 0.34% dose in the brain at 5 min and 0.28% dose in the brain at 60 min post-injection. The half-life for clearance of 99mTc-DADT-IPE(a) from rat brain was found to be more than 5 h. These results suggested that 99mTc-DADT-IPE(a) has characteristics which are suitable for cerebral perfusion imaging.