The present invention relates to novel l-aryl-4-methyl-[l,2,4]triazolo[4,3-a]- quinoxaline derivatives as inhibitors of phosphodiesterase 2 (PDE2) and to a lesser extent of phosphodiesterase 10 (PDE10) or as inhibitors of both, phosphodiesterases 2 and 10. The invention is also directed to pharmaceutical compositions comprising such compounds, to processes for preparing such compounds and compositions, and to the use of such compounds and compositions for the prevention and treatment of disorders in which PDE2 is involved, or disorders in which both PDE2 and PDE10 are involved, such as neurological and psychiatric disorders, and endocrinological or metabolic diseases. The present invention also relates to radiolabeled compounds which may be useful for imaging and quantifying the PDE2 enzyme in tissues, using positron- emission tomography (PET). The invention is also directed to compositions comprising such compounds, to processes for preparing such compounds and compositions, to the use of such compounds and compositions for imaging a tissue, cells or a host, in vitro or in vivo and to precursors of said compounds.La présente invention concerne de nouveaux dérivés de 1-aryl-4-méthyl-[1,2,4]triazolo[4,3-a]-quinoxaline comme inhibiteurs de phosphodiestérase 2 (PDE2) et dans une moindre mesure de phosphodiestérase 10 (PDE10) ou comme inhibiteurs des deux phosphodiestérases, 2 et 10. Linvention concerne également des compositions pharmaceutiques comprenant de tels composés, des procédés pour préparer de tels composés et de telles compositions et lutilisation de tels composés et de telles compositions pour la prévention et le traitement de troubles dans lesquels la PDE2 est impliquée ou de troubles dans lesquels à la fois la PDE2 et la PDE10 sont impliquées, tels des troubles neurologiques et psychiatriques, et des maladies endocrines ou métaboliques. La présente invention concerne également des composés radiomarqués qui peuvent être utiles en imagerie et pour la quantif
