Regional and whole-brain tryptophan-hydroxylating activity and serotonin turnover were investigated in portacaval shunted (PCS) rats using an in vivo decarboxylase inhibition assay. To saturate tryptophan hydroxylation with amino acid substrate, rats were administered a high dose of tryptophan 1 h prior to analysis of brain tryptophan, 5-hydroxytryptophan, serotonin, and 5-hydroxyindoleacetic acid. The analysis revealed, as expected, higher brain concentrations of tryptophan and 5-hydroxyindoles and increased serotonin synthesis rate in PCS rats as compared with shamoperated controls. Saturating levels of brain tryptophan were achieved in both PCS and sham animals after exogenous tryptophan administration. The tryptophan load resulted in increased brain serotonin turnover in all regions and in whole brain compared with rats that did not receive a tryptophan load. Tryptophan-loaded PCS rats showed increased brain serotonin turnover compared with tryptophan-loaded sham rats. Regionally, this supranormal tryptophan-hydroxylating activity was most pronounced in the mesencephalon-pons followed by the cortex. It is concluded that, at least in the PCS rat, brain tryptophan hydroxylation is an inducible process. Since it is known that brain tissue from PCS rats undergoes a redox shift toward a reduced state and that the essential cofactor tetrahydrobiopterin is active in tryptophan hydroxylation only when present in its reduced form, it is hypothesized that this is the reason for the supranormal tryptophan-hydroxylating activity displayed by the PCS rats. The hypothesis further suggests that alterations in tetrahydrobiopterin availability may serve as a mechanism by which brain tryptophan hydroxylation, and therefore serotonin turnover, can be regulated with high sensitivity in vivo.