Using Tris-acetate buffer rather than conventional phosphate buffer, it was possible to detect two distinct proteins capable of producing the neuroinhibitory brain metabolite kynurenic acid (KYNA) from L-kynurenine in human brain tissue. The two kynurenine aminotransferases (KATs), arbitrarily termed 'KAT I' and 'KAT II', could be physically separated by isoelectric focussing on a pH 3-10 Ampholine gradient, and, more completely, by differential elution from a DEAE-Sepharose column. KAT I showed a pronounced preference for pyruvate as a co-factor and had a pH optimum of 9.6. In contrast, KAT II was virtually equally active when either pyruvate or 2-oxoglutarate were used as the aminoacceptor, and its pH optimum was 7.4. Moreover, KAT I and KAT II differed with regard to their sensitivity to amino acids and as the aminoacceptor, and its pH optimum was 7.4. Moreover, KAT I and KAT II differed with regard to their sensitivity to amino acids and kinetic characteristics. The existence of two separate enzymes capable of producing KYNA in the human brain raises the question if and to what extent each of the enzymes regulates the cerebral synthesis of KYNA and its possible role as a modulator of excitatory amino acid receptor function.