text: N(5)-Substituted H(4)biopterin derivatives were not oxidized to products serving as substrates for dihydropteridine reductase and,depending on the substituent, were competitive inhibitors of phenylalanine hydroxylase: N(5)-methyl- and << N(5)-hydroxymethyl H(4)biopterin >> inhibited [[ phenylalanine hydroxylase ]], whereas N(5)-formyl- and N(5)-acetyl H(4)biopterin had no effect.