Zinc, at low levels, has several basic housekeeping functions in metalloenzymes, transcription factors, immunoregulation, growth, and cytoprotection, displaying antioxidant, anti-apoptotic, and anti-inflammatory roles. At high levels, however, the metal can be highly toxic. The aim of this work is to investigate the toxic effect of zinc on antioxidant status and stress proteins in the gills of the brown mussel Perna perna exposed for 48 h to zinc chloride (zinc) at 10, 30 and 100 microM. Glutathione reductase (GR) activity was drastically reduced at 30 and 100 microM zinc. At the lower levels, i.e. 10 microM zinc, antioxidant defenses were up-regulated, as were glutathione levels and the activities of glutathione peroxidase and catalase, in spite of the absence of effect on glutathione S-transferase and glucose 6-phosphate dehydrogenase activity. At the higher tested concentration of 100 microM zinc, oxidative stress was apparent as reflected by the increased lipid peroxidation end products and decreased protein thiol and glutathione levels, associated with an inability to up regulate antioxidant defenses. Using 30 microM zinc, higher gill rhodamine B efflux was observed, indicating an activation of multixenobiotic resistance (MXR) activity, which is reinforced by increased immunoreactive P-glycoprotein detection. Zinc also increased the HSP60-immunoreactive protein, whereas the HSP70-immunoreactive protein remained unchanged. Overall, the results indicate that zinc toxicity -- at higher levels -- may be connected to a strong inhibition of GR activity, and related to the pro-oxidative state found. Mussels showed an adaptive-like response to 10 microM zinc by increasing antioxidant defenses. Increased P-glycoprotein and HSP60 expression, and rhodamine B efflux were also remarkable features in the gill response to zinc.