Treatment of many inherited liver enzyme deficiencies requires the removal of toxic intermediate metabolites from the blood of affected individuals. We propose that circulating toxins can be adequately cleared and disease phenotype influenced by enzyme expressed in tissues other than the liver. Phenylalanine hydroxylase (PAH) activity was constitutively expressed in skeletal and cardiac muscle of transgenic mice which carried the PAH cDNA under the transcriptional control of the mouse muscle creatine kinase promoter. Muscle PAH-expressing mice were bred to liver PAH-deficient, hyperphenylalaninemic mice to yield progeny that lack PAH activity in liver but express PAH in muscle. These mice exhibited hyperphenylalaninemia at baseline, but serum phenylalanine levels decreased significantly when the mice were supplemented with tetrahydrobiopterin (BH4), a required cofactor for PAH. This is the first demonstration that a liver-specific enzyme, when expressed in a heterologous tissue and supplied with necessary cofactors, can effectively clear toxic metabolites from the circulation of individuals with inherited enzyme deficiency. This result suggests that gene therapy targeted to heterologous tissues, such as muscle, will be effective in the treatment of selected inborn errors of metabolism.