Autoradiography: Intravenous (2--14C)-Tetrahydrobiopterin (BH4; 5 muCi, spec. act. 50 Ci/mol) is incorporated into liver and kidney of the rat. The gastrointestinal tract is labelled possibly due to biliary excretion of BH4 and its metabolites. BH4 and its metabolites obviously do not pass the blood brain barrier. Estimation of Crithidia active material shows a regional and subcellular distribution of the coenzyme. The highest levels were found in the liver (4.6 mumol/kg), and in spleen (2.3 mumol/kg). In the brain, highest coenzyme concentration is in the hypothalamic region with 0.49 mumol/kg as compared with 0.25 mumol/kg for total brain value. After subcellular fractionation, biopterins (expressed as Crithidia activity) are located mainly in the soluble fraction of liver (61.3 +/- 6.7) and kidney (89.1 +/- 14.0), whereas subcellular fractions of brain show additional higher percentage distribution in lysosomal (23.4 +/- 3.3) and microsomal (24.9 +/- 5.2) fractions due to their synaptosomal content as shown by density gradient centrifugation (values in brackets: percentage +/- SD).