Tetrahydrobiopterin (BH4) is the redox cofactor for the aromatic amino acid hydroxylases such as phenylalanine hydroxylase. At least five enzymes are known to be involved in BH4 biosynthesis and regeneration. A deficiency in several of the BH4 metabolic enzymes causes variant types of hyperphenylalaninemias in man. Recently, we cloned and expressed the human cDNAs for two of the BH4 enzymes, the 6-pyruvoyl-tetrahydropterin synthase and the pterin-4 alpha-carbinolamine dehydratase (gene symbols PTS and PCD/DCOH, respectively). In this report, we localized the two genes on the human chromosomes by in situ hybridization. The PTS gene was mapped to the chromosomal region 11q22.3-q23.3, and the PCD/DCOH gene was mapped to the 10q22 band of the genome.