This study deals with the occurrence of oxidative stress and antioxidant response in the Antarctic intertidal limpet Nacella concinna, as an effect of temperature increments and H 2 O 2 exposure under controlled laboratory conditions. Experiments were designed to simulate transient conditions of increased T and/or H 2 O 2 accumulation met by the limpets in intertidal rockpool habitats [5]. Specimens were collected at Jubany Station, South Shetland Islands, transferred to the Alfred-Wegener Institute, Bremerhaven and maintained in seawater aquaria at 0°C. Different groups of animals were acclimated at 4 and 9°C for 24–48 h (controls at 0°C). The effect of starvation was studied at 0°C and of H 2 O 2 exposure at 4°C. Temperature acclimation above 0°C resulted in a progressive alteration of the lysosomal compartment in digestive gland cells, as shown by cytochemical analyses (lipofuscin and neutral lipid accumulation and lysosomal membrane destabilization). Concurrently, real activities of the antioxidant enzymes superoxide dismutase (SOD) and catalase (i.e. measured at the respective experimental temperature or calculated by means of previously determined Q 10 values) increased in gills and digestive gland tissues. Measurements of intracellular pH at the different temperatures showed a rise from pH 7.21 at 0°C to 7.36 at 9°C. These changes in pH are indicated to increase SOD activity by approximately 10% in both kinds of tissue at 9° as compared to 0°C. H 2 O 2 exposure at 4°C produced physiological alterations at the systemic (lowered O 2 consumption) and at the cellular levels (enhanced lysosome damage). Starvation induced lysosomal alterations in animals kept at 0°C and inhibited CAT activation under H 2 O 2 exposure at 4°C. The complex of data suggests that when Nacella migrates to intertidal levels during the Austral Spring it experiences oxidative stress which induces an antioxidant response, which is facilitated by higher temperatures and increasing intracellular pH and the exploitation of intertidal food resources. Yet, the occurrence of cellular damage and systemic alterations shows that the limpets approach their boundaries of physiological tolerance during prolonged exposure to higher temperatures and H 2 O 2 in intertidal habitats.