This study aimed at elucidating whether organic and inorganic forms of mercury contamination differently affect HSP and MXR cyto-protective responses in Mytilus galloprovincialis. Changes of relative transcript levels were assessed in digestive glands of mussels exposed to 0.75 mu M CH3Hg+ or Hg2+ for a maximum of 6 days. CH3Hg+ inhibited MgHSP70 and induced MgHSC70 expression. As previously reported Hg2+ stimulated MgHSP70 expression and induced MgHSC70 transcripts only after 6 days. The effect of CH3Hg+ or Hg2+ on the MXR system was investigated after partial sequencing of Pgp, Mrp2, and Mvp cDNAs. The multiple alignments showed a high sequence homology with the human and the available bivalve MXR-related proteins. All transcripts were up-regulated by CH3Hg+ while transiently up-regulated by Hg after 8 h of exposure, and down-regulated thereafter. Present results suggest that the HSP70 and MXR systems may be simultaneously modulated by the same stress stimulus, and that different chemical forms of contamination evoke different cytoprotective responses. (c) 2006 Elsevier Inc. All rights reserved.