The pteridine tetrahydrobiopterin (BH4) is an essential cofactor in the biosynthesis of dopamine, (nor)epinephrine, serotonin and nitric oxide (NO). Furthermore, BH4 has a direct influence on release mechanisms of these neurotransmitters and on serotonin receptor binding activity immunology. The synthesis of BH4 is stimulated by interferon-gamma and hence there is a close relationship with the immune system HPA-axis. In animal experiments it was also found that the hypothalamus–pituitary–adrenal axis influences the pteridine metabolism. In clinical studies, so far, no evidence has been found for this relationship diseases. A congenital biopterin deficiency results in atypical phenylketonuria with severe neuropsychiatric symptoms. In several neurological diseases, such as Parkinson's disease, decreased levels of BH4 are found depression. Since 1984 there have been reports on decreased biopterin and increased neopterin levels in urine and plasma of depressed patients. Conflicting results have also been found, however, due probably to methodological problems therapy. Until now, oral administration of BH4 to depressed patients has been performed by two investigators, which resulted in mainly temporal clinical improvement discussion. Understanding of biochemical mechanisms in which pteridines are involved may contribute to our knowledge of the pathogenesis and treatment of affective disorders. This paper aims to provide an overview of the relevant literature and warrant for further research on this intriguing compound.