The invention provides a new method of treating diseases or conditions characterized by reduced dopamine D1 receptor signaling activity, such as Parkinson's disease, depression, and cognitive impairment of schizophrenia, comprising administering an effective amount of a 1,3,5,-substituted, 6,7-dihydro-1H-pyrazolo[4,3-d]pyrimidin-7-one to a patient in need thereof.