Measurement of tetrahydrobiopterin in cerebrospinal fluid requires careful handling of samples during storage and analysis. Addition of dithioerythritol and deoxygenation of the mobile phase with helium prevents breakdown of tetrahydrobiopterin during chromatography. Tetrahydrobiopterin in cerebrospinal fluid is unstable at room temperature, 100% being lost within 3.5 h, this breakdown does not generate equivalent quantities of dihydrobiopterin and biopterin. Addition of dithioerythritol and diethylenetriaminepenta-acetic acid to cerebrospinal fluid prevents breakdown of tetrahydrobiopterin for 6 mth at -70 degrees C and for up to 5 h at 4 degrees C. At room temperature less than 5% of tetrahydrobiopterin was lost after 2 h.