A series of five 3-methyl-1-phenyl-4-substituted pyrazolo[3,4-d]pyrimidin-6-ones(I a-e) and five 3-methyl-1-phenyl-4-substituted pyrazolo [3,4-d]pyrimidin-6-thiones (II a-e) were prepared and some of them were subjected to aminolysis as well as hydrazinolysis. The reaction of the 6-chloro derivatives (IV) with amino acids to give pyrazolo pyrimidine derivatives (IX) was investigated. A few of the pyrimidin-6-thiones were cyanoethylated under drastic conditions to give (XI), and some of them were also hydroxyethylated (IX).