A series of 6-phenylpyrazolo[3,4-d]pyrimidones is described which are specific inhibitors of cGMP specific (type V) phosphodiesterase. Enzymatic and cellular activity as well as in vivo oral antihypertensive activity are evaluated. A n-propoxy group at the 2-position of the phenyl ring is necessary for activity. A series of products substituted at the 5-position in addition to the 2-n-propoxy was prepared and evaluated. This position can accommodate many unrelated groups. Amino derivatives were very potent but lacked metabolic stability. Substitution by carbon-linked small heterocycles provided both high levels of activity and stability. Cellular activity very often correlated with in vivo activity. Among the compounds, 1,3-dimethyl-6(2-propoxy-5-methanesulfonamidophenyl)-1,5-dihydr opyrazolo[3,4-d]pyrimidin-4-one (38) and 1-ethyl-3-methyl-6-(2-propoxy-5-(4-methylthiazol-2-yl)phenyl -1,5-dihy dropyrazolo[3,4-d]pyrimidin-4-one (59) displayed outstanding in vivo activities at 5 mg/kg/os and good metabolic stabilities.