The present invention is directed to 6-alkyl dihydropyrazolopyrimidinone compounds of formula (I) which are useful as therapeutic agents for the treatment of central nervous system disorders associated with phosphodiesterase 2 (PDE2). The present invention also relates to the use of such compounds for treating neurological and psychiatric disorders, such as schizophrenia, psychosis, Parkinson' s disease, Parkinson' s disease dementia (PDD), or Huntington' s disease, and those associated with striatal hypo-function or basal ganglia dysfunction.