5-HT1A receptor is associated with a variety of pathophysiology of neuropaychiatrlc disorders. Ac-cordingly, we have synthesized a new 5-HT1A receptor ligand (HYNIC-MPP4) and labeled it with 99mTc using N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) as coligand. 99mTc-HEDTA/HYNIC-MPP4 was prepared under pH 6 at room temperature. Biodistribution of 99mTc-HEDTA/HYNIC-MPP4 in normal Hipp/CB was 3.1 at 2 rain p.i. and increased to 4.4 at 60 min p.i. After blocking with 8-hydroxy-2-(dipro-potent agent for 5-HT1A receptor imaging.