In mammalian peripheral organs, 3‐hydroxy‐anthranilic acid oxygenase (3HAO), catalyzing the conversion of 3‐hydroxyanthranilic acid to quinolinic acid, constitutes a link in the catabolic pathway of tryptophan to NAD. Because of the possible involvement of quinolinic acid in the initiation of neurodegenerative phenomena, we examined the presence and characteristics of 3HA0 in rat brain tissue. A simple and sensitive assay method, based on the use of [carboxy‐¹⁴C]3‐hydroxy‐anthranilic acid as a substrate, was developed and the enzymatic product, [¹⁴C]quinolinic acid, identified by chro‐matographic and biochemical means. Kinetic analysis of rat forebrain 3HAO revealed a K_m of 3.6 ± 0.5 μM for 3‐hydroxyanthranilic acid and a V_maxof 13.1± 9.5 pmol quinolinic acid/h/mg tissue. The enzyme showed pronounced selectivity for its substrate, since several substances structurally and metabolically related to 3‐hydroxyanthranilic acid caused <25% inhibition of activity at 500 μM. Both the Fe²⁺ dependency and the distinct subcellular distribution (soluble fraction) of brain 3HAO indicated a close resemblance to 3HAO from peripheral tissues. Examination of the regional distribution in the brain demonstrated a 10‐fold variation between the region of highest (olfactory bulb) and lowest (retina) 3HAO activity. The brain enzyme was present at the earliest age tested (7 days postnatum) and increased to 167% at 15 days before reaching adult levels. Enzyme activity was stable over extended periods of storage at — 80°C. Taken together, these data indicate that measurements of brain 3HA0 may yield significant information concerning a possible role of quinolinic acid in brain function and/or dysfunction.