Tetrahydrobiopterin (BH(4)) is widely used as a therapeutic agent in patients with BH(4) deficiencies and mild forms of phenylketonuria (PKU) and there is an increasing need for the measurement of its plasma concentrations in patients with cardiovascular disorders. We measured BH(4) and total biopterin in dithioerythritol (DTE) pretreated plasma from four adults after oral administration of BH(4) (2, 10, and 20mg/kg body weight) using the differential iodine oxidation method. About 80% (range 64.8-92.2% ) of total biopterin was found as BH(4) when analyzed immediately after blood sampling. Compared with ascorbic acid as an antioxidant, DTE was more protective against oxidation of BH(4), particularly in samples stored over a period of 8 months. Without antioxidant (DTE or ascorbic acid) almost no BH(4) was detected. Furthermore, BH(4) and total biopterin were measured at different time intervals (up to 33 h after oral administration) and pharmacokinetic parameters T(max) (1-4h), C(max) (258.7-259.0 nmol/L biopterin at a dosage of 10mg/kg), and area under the curve (AUC=1708-1958 nmol(*)h/L up to T=10h) were estimated. The elimination half-life time was calculated to be 3.3-5.1h. Doubling the BH(4) dosage to 20mg/kg resulted in 60% higher AUC while sublingual BH(4) application (2mg/kg) resulted in 58-76% higher BH(4) plasma concentrations when compared with oral administration. These preliminary data suggest that in patients with BH(4) cofactor defects and BH(4)-responsive phenylalanine hydroxylase deficiency, BH(4) should be given in at least two to three daily doses and that sublingual administration may lower the required BH(4) dosage and subsequently the cost of treatment. Due to inter individual differences in pharmacokinetic properties, in some patients with hyperphenylalaninemia and mild PKU plasma BH(4) levels may be not high enough to fully activate the liver phenylalanine hydroxylase and thus lower blood phenylalanine levels. Assessment of plasma BH(4) or total biopterin concentrations may be a good way to control the efficacy of the loading test.