The present invention provides 1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-ones and 1,5-dihydro-4H-pyrazolo[4,3-c]pyridin-4-ones of formula (I) as PDE1 inhibitors and their use as a medicament, in particular for the treatment of neurodegenerative disorders and psychiatric disorders.本發明提供了作為PDE1抑制劑的具有化學式(I)之1,5-二氫-4H-吡唑并[3,4-d]嘧啶-4-酮和1,5-二氫-4H-吡唑并[4,3-c]吡啶-4-酮及其作為藥劑特別是用於治療神經退行性障礙和精神障礙之用途。