The effects of temperature (0, 5, 10, 15 and 20 °C) and of anoxia (at 5°C) on extracellular Mg2+ concentration ([Mg2+](e)), intracellular pH (pHi) and ATP and lactate levels were investigated in intermoult adults of the common shrimp Crangon crangon. All animals caught in summer (summer animals) showed a slight but significant increase in [Mg2+](e) at low temperatures. In contrast, at every temperature tested, a few of the animals caught in winter (winter animals) showed elevated [Mg2+](e) during short-term (4 h) but not during long-term (6 days) incubations. The reasons for the overshoot in Mg2+ concentrations in individual animals remain unexplained, but a protective effect of extracellular Mg2+ on intracellular pH and on ATP concentrations was visible at haemolymph Mg2+ concentrations above 15 mmol-1. The influence of high extracellular [Mg2+] on pHi and intracellular ATP and lactate levels under normoxic and anoxic conditions was tested using an incubation medium containing 150-250 mmol l-1 Mg2+. When haemolymph Mg2+ levels were manipulated by exposure of the animal to high levels of Mg2+ in the external medium, animals with a haemolymph [Mg2+] below the threshold concentration of 15 mmol l-1 had significantly lower values of intracellular pH than animals with haemolymph [Mg2+] above 15 mmol l-1. In addition, the elevation of haemolymph [Mg2+] by incubation in high-[Mg2+] water prevented the drop in phi and the rise in lactate levels induced by anoxia. The protective effect of high levels of extracellular Mg2+ did not depend upon the [Ca2+]/[Mg2+] ratio but only on [Mg2+](e). However, experiments with isolated muscle tissues showed no dependence of muscle intracellular pH on [Mg2+](e) under both normoxic and anoxic conditions, leading to the conclusion that the protective effect is evoked via a central, possibly anaesthetising, effect of high [Mg2+](e). The dependence of phi and muscle [ATP] on extracellular [Mg2+] resembles the protective effect of high Mg2+ levels on the post-ischaemic mammalian heart.