Both basic and clinical studies have indicated that the 3', 5'-cyclic adenosine monophosphate (cAMP) system plays critical roles in several brain diseases, particularly in mood disorders and drug addiction. cAMP is synthesized from adenosine 5'-triphosphate (ATP) by adenylyl cyclase and metabolized by cyclic nucleotide phosphodiesterases (PDEs). Among components of the cAMP pathway, PDE4 appears to be critical for antidepressant effects. 4-[3-(cyclopentoxyl)-4-methoxyphenyl]-2-pyrrolidone (rolipram) is an inhibitor of PDE4. As a positron emission tomography (PET) brain imaging agent, rolipram has good properties such as high affinity of 1-2 nM and appropriate lipophilicity (Log P) of ~3. A rat study gave an estimation of low radiation absorbed doses of the active enantiomer (R)-[11C]rolipram. ciociWEge quality. Therefore, R-[11C]rolipram is a promising PET ligand. However, radiation absorbed doses have not been estimated from human whole body imaging studies and a method to measure binding of (R)-[11C]rolipram in human brain has not been established.