In this research, three lines of investigation were pursued. (1) In vivo effects of temperature and H2O2 were evaluated in the limpet Nacella concinna. Production of reactive oxygen species was assessed with the dihydrorhodamine 123 fluorescent probe, lysosome alterations with histochemical techniques, and antioxidant responses through Superoxide dismutase and catalase assays. Data suggest that Nacella spring migrations to intertidal levels, where rises in temperature occur, induce oxidative stress and antioxidant response. (2) In the scallop Adamussium colbecki, the sarcoplasmic reticulum Ca2+ ATPase was characterized and heavy metal effects were assessed. An IC50 of 0.9 μ? was found for Hg2+ and of 3 μ? for Cd2+. (3) Effects of Cu2+, Cd2+, and Hg2+ (2.5 μ?), and of temperature on Ca2+ homeostasis and cell viability were assessed in the ciliate Euplotes focardii. Ca2+ homeostasis and cell viability were altered by Hg2+ at a temperature of 0° C and above, and by Cu2+ and Cd2+ at 10° C and above. Temperature alone was effective at 20° and 30° C, suggesting that rises in temperature cause an increased cell sensitivity to environmental contaminants. © 2000 Taylor & Francis Group, LLC.