Due to low metabolic rate, slow uptake kinetics, slow recovery rates, short food chains, longer acute response times in polar species compared to temperate species, use of temperate species to assess toxicity to polar species may be difficult. As such polar species may appear less sensitive than temperate species, due to higher acute toxicity measurements. There have, however, apparently been few attempts to determine whether temperate effect levels are in fact applicable to polar biota. In an attempt to investigate potential differences in acute toxicity and biotransformation, we used two pelagic copepod species Calanus finmarchicus and C. glacialis which are both present and highly important species in the Northern Atlantic Ocean food web. Whereas C. finmarchicus is found in most of the Northern Atlantic, C. glacialis is truly Arctic. Acute toxicity was assessed after exposure to a model toxicant, ionic mercury, for 96 hours for both C. finmarchicus and C. glacialis. Culture C. finmarchicus (cultured for 19 generations at 8-10°C at SINTEF/NTNU Sealab) and net-hauled C. glacialis from Kongsfjorden (Kings Bay Laboratory, Svalbard, 2°C) were used with identical experimental design (7 ind x 6 conc x 3 repl) for acute toxicity determination (LC50). Body residue of mercury after the acute toxicity test was determined. We also conducted sublethal experiments where copepods were sampled for stress gene expression after being exposed to different concentrations (0.5, 5 and 50% of LC50 concentration) and times (12, 24 and 48 hours). Preliminary results of these comparative experiments showed that similar responses were observed for both species and that for ionic mercury there were no clear differences in response between a more temperate and a polar copepod species.