Dominantly inherited guanosine triphosphate (GTP)-cyclohydrolase deficiency, otherwise known as Segawa's disease or dopa-responsive dystonia, has a wide spectrum of phenotypic expression ranging from asymptomatic to very severe. Penetrance is more frequent in women as compared with men, and there is a variable occurrence of diurnal variation in symptom intensity. Biochemical characterization of the disease has demonstrated lower cerebrospinal fluid levels of tetrahydrobiopterin (BH4), neopterin, and homovanillic acid and low levels of tyrosine hydroxylase protein in the striatum. To investigate the pathophysiology, we have begun to characterize biogenic amine and BH4 metabolism in the GTP cyclohydrolase deficient hph-1 mouse. The data show low brain levels of BH4, catecholamines, serotonin, and their metabolites together with low levels of tyrosine hydroxylase protein within the striatum. The hph-1 mouse therefore provides a good model system in which to study the human disease.