The dithiocarbamates are well known for their antioxidant properties and effects on cellular transcriptional events. For example, pyrrolidine dithiocarbamate (PDTC) is widely used as an inhibitor of nuclear factor kappa B (NFκB) and this, or related compounds may have therapeutic potential in inhibiting atherosclerosis. However, the precise molecular mechanisms through which PDTC could elicit antioxidant or cell signaling effects in a cellular setting remain unclear. Furthermore, the mechanisms for the effects of PDTC on NFκB are likely to involve inhibition of binding of the transcription factor to DNA rather than an effect on the activation process as first proposed. In relation to pharmacological applications of such compounds, little is known of their interaction with endothelial cells, the anticipated site of action for inhibition of vascular related diseases. Until recently, PDTC was generally classified as an antioxidant but evidence for pro-oxidant effects have been reported. In this study, we have addressed this issue in bovine aortic endothelial cells and identified two mechanisms through which PDTC can exert antioxidant effects. At low concentrations (0-25 μM), PDTC induces a concentration dependent increase in cellular GSH levels through the increased activity of γ-glutamylcysteine synthetase. At higher concentrations, GSH oxidation and apoptotic cell death occur. Using 2,3 dimethoxy-1,4-napthoquinone (DMNQ) as an intracellular generator of superoxide radicals, we find PDTC (10 μM) protects against the cytotoxicity of this agent through a GSH-independent mechanism.