The plasma total biopterin and tetrahydrobiopterin (BH4) levels of 24 psychiatric patients were measured in the symptomatic phase, under no influence of psychotropic drugs as long as possible, and were compared with those of normal controls. The significant increase in total biopterin levels was observed only in the plasma of the patients with affective disorders or panic disorder in the depressive mood. A significant decrease in BH4 levels was observed only in the plasma of the patients with depression, and an increase in BH4 levels was observed only in the patients with hypomania. The changes of these biopterin levels in plasma could be a disease effect or a phase effect.