The present invention relates to (1- or 2- and/or 5 and/or 7-substituted)-(3-oxy)-(4H, 4-imino, 4-thioxo or 4-oxo)-pyrazolo[3,4-d]pyrimidin-6-ones, e.g., a compound of formula II as described below, processes for their production, their use as pharmaceuticals and pharmaceutical compositions comprising them. Of particular interest are novel compounds useful as inhibitors of phosphodiesterase 1 (PDE1), e.g., in the treatment of diseases involving disorders of the dopamine D1 receptor intracellular pathway, such as Parkinson's disease, depression, narcolepsy, damage to cognitive function, e.g., in schizophrenia, or disorders that may be ameliorated through enhanced progesterone-signaling pathway, e.g., female sexual dysfunction.