Provided is method for improving depression of a patient by administering to a patient suffering from depression an effective dose of sepiapterin or a pharmaceutically acceptable salt of sepiapterin in an amount from 0.1 to 100 mg/kg of body weight per day. The sepiapterin or salt of sepiapterin entering the patient's bloodstream and passing the patient's blood-brain barrier to enter a neuron, and the sepiapterin or salt of sepiapterin converting to tetrahydrobiopterin in a neuron of the brain and the tetrahydrobiopterin facilitating biosynthesis of an aromatic monoamine in the patient's brain to provide an elevated level of an aromatic monoamine in the patient's brain to improve depression of the patient
