A series of 2-amino-S-aryl cysteine S,S-dioxides have been synthesised and shown to inhibit kynureninase an important enzyme in the biosynthesis of the known excitotoxic moiety quinolinic acid. The most potent of these, 2-amino-5-methyl-S-phenyl cysteine S,S-dioxide 6d, inhibits interferon-gamma induced synthesis of quinolinic acid in human macrophages.