Intraperitoneally injected [(14)C]biopterin (B), dihydrobiopterin (BH2), and tetrahydrobiopterin (BH4) penetrated the brain rapidly, but in amounts sufficient to represent only a minor source of supply. Unlike isobiopterin, B was rapidly reduced to BH2 in brain. The distribution of BH4 and BH2, but not of B, could be correlated with the tryptophan and tyrosine hydroxylase activity of various cerebral areas. In the whole brain, the sizes of pools were 0.117 for B, 0.204 for BH2, and 0.341 μg/g for BH4, while the cerebral turnover rates of B, BH2, and BH4 were 0.25, 0.43, and 0.71 nmol/g per h, respectively. From birth through development, the cerebral levels of B remained constant, whereas the levels of the reduced biopterins increased. After subcellular fractionation, 65% of the biopterins (B, BH2, and BH4) were recovered from the supernatant. Of the organelles, microsomes contained the largest concentration of pterins. About 1/6 of all pterins in the brain was present in the synaptosomes.