We identified an age effect on biopterin and biogenic amine metabolites in the cerebrospinal fluid from 56 neurologically normal patients, aged 1 mo to 80 y. The levels of total and reduced forms of biopterin, homovanillic acid, and 5-hydroxyindoleacetic acid were found to peak during the first year of life. The levels of these metabolites then gradually decreased, plateauing at the age of 20 y. We found significant correlations between biopterin, homovanillic acid, and 5-hydroxy-indoleacetic acid.