To study the regulation of the synthesis of quinolinic and kynurenic acids in vivo, we evaluated (a) the metabolism of administered kynurenine by measuring the content of its main metabolites 3-hydroxykynurenine, anthranilic acid, and 3-hydroxyanthranilic acid in blood and brain of mice; (b) the effects of (m-nitrobenzoyl)alanine, a selective inhibitor of kynurenine hydroxylase and of (o-methoxybenzoyl) alanine, a selective inhibitor of kynureninase, on this metabolism; and (c) the effects of (o-methoxybenzoyl)alanine on liver kynureninase and 3-hydroxykynureninase activity. The conclusions drawn from these experiments are (a) the disposition of administered kynurenine preferentially occurs through hydroxylation in brain and through hydrolysis in peripheral tissues; (b) (m-nitrobenzoyl)alanine, the inhibitor of kynurenine hydroxylase, causes the expected changes in brain kynurenine metabolism, such as a decrease of 3-hydroxykynurenine, and an increase of kynurenic acid; and (c) (o-methoxybenzoyl)alanine, the kynureninase inhibitor, increases brain concentration of the cytotoxic compound 3-hydroxykynurenine, and unexpectedly does not reduce brain concentration of 3-hydroxyanthranilic acid, the direct precursor of quinolinic acid. Taken together, the experiments suggest that the systemic administration of a kynurenine hydroxylase inhibitor is a rational approach to increase the brain content of kynurenate and to decrease that of cytotoxic kynurenine metabolites, such as 3-hydroxykynurenine and quinolinic acid.