Kynurenine aminotransferase (KAT) is an enzyme responsible for synthesis of kynurenic acid (KYNA), a well established neuroprotective and anticonvulsant agent, involved in synaptic transmission and implicated in the pathophysiology of schizophrenia, Huntington's disease and other neurological disorders. We have shown previously that kat2-/- mice had lower hippocampal KYNA levels and were more hyperactive than wild-type mice. However, these abnormalities occur early and are transitory coinciding with restoration of KYNA levels, suggesting that compensatory changes or ontogenetic expression of another unknown homolog may account for the normalization of KYNA levels in the adult kat2-/- mice brain. Here, we report the isolation of a novel KAT molecule, kat3, from mouse and human brain cDNA libraries. The encoded 454 amino acids of human KAT III share 64.8% similarity to that of KAT I and 30.1% to KAT II. Northern blot analysis demonstrated that kat3 mRNA is widely expressed but with higher expression levels in liver, kidney, heart, and neuroendocrine tissues. RT-PCR and Northern analysis showed that kat3 expression starts as early as postnatal day (PND) 7 and peaks in adult. The mRNA level of kat3 and kat1 when measured together is significantly higher at PND 60 in kat2-/- mice than those of wild-type mice indicating possible co-regulation of expression levels. RNA-interference (RNAi) directed towards transcripts for either R03A10.4 or F28H6.3 in Caenorhabditis elegans which are kat1 and kat3 orthologs, respectively, did not result in any gross abnormalities. Our results show that upregulation of kat3 and kat1 may be responsible for the phenotypic rescue on kat2-/- mice.