In the present study, we investigated age-related changes in pteridine levels and enzymatic activity responsible for tetrahydrobiopterin biosynthesis in mouse tissues. Until about 15 weeks after the birth, the remarkable change of tetrahydrobiopterin (BH4) was observed in all tissues tested. Between 20 and 50 weeks after the birth, pteridines levels were almost constant in all of the tissues. Total biopterin levels were decreased and levels of pterin and neopterin were increased in the period exceeding 50 weeks in all of the tissues. Activities of guanosine triphosphate (GTP) cyclohydrolase I, pyrvoyltetrahydropterin synthase, and the production of BH4 were recognized by specific biochemical assays, and we investigated the age-related changes in mouse tissues. The alteration of these enzymatic activities was indicated to be similar to that described in the change of pteridine levels.