Introduction and Objectives: Glutathione (GSH) is a major intracellular amino thiol which participates in protein synthesis and the detoxification of free radicals (1). GSH metabolism is a complex compartmentalised inter-organ process, relying on the bioavailability of homocysteine (Hcy) and cysteine (Cys). GSH is degraded by membrane-bound gamma-glutamyltranspeptidase (gGT) to form cysteinylglycine conjugates (CysGly) that are involved in the de novo generation of Cys to maintain intracellular GSH levels during oxidative stress (2). GSH depletion is a consistent feature of the progression of a mild to severe attack of acute pancreatitis (AP), and in order to understand glutathione homeostasis during an attack we investigated the temporal relationship between plasma concentrations of the amino thiols Hcy, Cys and CysGly, total erythrocyte GSH (GSHt) concentration, and the clinical course of AP. Methods: Patients with AP were recruited within 24hrs of onset of pain. Peripheral blood (plasma) was analysed at 24hrs and 72hrs after the onset of abdominal pain for Hcy, Cys, and CysGly concentrations using HPLC with fluorescence detection (3). Serum gGT and erythrocyte GSHt concentrations were determined using sandwich ELISA. The severity of disease was classified into mild and severe attacks based on the Atlanta criteria. Results: In total 41 patients with AP (mild 30, severe 11) were included. Overall Cys, Hcy and CysGly and gGT concentration were similar in the two groups at 24-hrs, although at 72hrs, CysGly levels were markedly higher (p=0.002), and erythrocyte GSHt significantly lower (p=0.033) in severe attacks compared to a mild attack. During the course of the disease, significant increases were observed in amino thiol (p < 0.01) and GSHt (p=0.004) concentrations for mild attacks, in whom plasma Cys correlated strongly with Hcy (r = 0.77, p<0.001), and gGT activity [r = 0.75, p<0.001). No such relationship existed in severe AP in which increases in Hcy (p=0.011) were not paralleled by an increase in Cys. Furthermore significant decreases in erythrocyte GSHt (p=0.004) and gGT activity (p=0.012) were observed. Levels of the GSH metabolite CysGly markedly increased (p=0.018), and this strongly correlated with erythrocyte GSHt depletion (r = 0.99, p=0.01). Conclusion: Glutathione depletion associated with severe acute pancreatitis is characterised by an increased rate of GSH degradation, as shown by increased CysGly levels. Our results also indicate that impaired GSH synthesis occurs despite adequate Cys supply.