A highly sensitive and simple assay for the activity of GTP cyclohydrolase I (EC 3.5.4.16) was established using a newly developed radioimmunoassay. D-erythro-7,8-Dihydroneopterin triphosphate formed from GTP by GTP cyclohydrolase I was oxidized by iodine and dephosphorylated by alkaline phosphatase to D-erythro-neopterin, and quantified by a radioimmunoassay for D-erythro-neopterin. This method was highly sensitive and required only 0.2 mg of rat liver tissues for the measurement of the activity. It was reproducible and can be applied for the simultaneous assay of many samples. The activity of GTP cyclohydrolase I was measured in several rat tissues. For example, the enzyme activity in rat striatum (n = 5) was 13.7 +/- 1.5 pmol/mg protein per hour (mean +/- SE), and agreed well with those obtained by high-performance liquid chromatography with fluorescence detection. The activity in the autopsy human brains (caudate nucleus) was measured by this new method for the first time. The activity in the caudate nucleus from parkinsonian patients (n = 6) was 0.82 +/- 0.56 pmol/mg protein per hour which was significantly lower than the control value, 4.22 +/- 0.43 pmol/mg protein per hour (n = 10).