The exchange rates of radioactive magnesium in various tissues of the rat were compared in normal and hypothermic animals ( 20° C}. The temperature effect was similar to that measured in isolated rat muscles incubated at 37° and 20° C, and the Q10 was 1. 7 in each case. Frog muscles were incubated at various temperatures, and the Q10 of Mg28 influx was calculated to be 1. 8; the Q10 of the concurrent efflux was 1. 2. These data suggest that the magnesium concentration in the cells is maintained by an active transport mechanism, and that the rise in serum magnesium in hypothermia is in part due to inhibition of this transport mechanism. The exchange rates of radioactive magnesium are less in the tissues of cold adapted rats than in normal controls, and the difference applies principally to the more rapid components of the exchange. Magnesium uptake by the rat liver is increased by administration of glucose and insulin, but this is evidently not a stoichiometric relationship between liver magnesium and liver glycogen.