SUMMARY1. When cardiac preparations are rewarmed following prolonged hypothermia a transient hyperpolarization occurs in K-containing media. This hyperpolarization is correlated with the active Na efflux. It might be due to electrogenic Na pumping or to extracellular K depletion brought about by the activity of an electroneutral Na-K exchange pump. In order to distinguish between these mechanisms the effect of various extracellular K concentrations ([K]O) on the membrane potential ofguineapig atria was studied before and after hypothermia. 3. The membrane hyperpolarized transiently after hypothermia beyond the potential observed before cooling. Maximal values of about -94 mV were obtained during rewarming in solutions containing 0-4-2-7 mM-K. The membrane potential was significantly lower (-88 mV) in K-free media. It was also diminished at [K]o higher than 2-7 mm and was measured to be -74 mV at 10-8 mM-K.4. The hyperpolarization of the cell membrane during the first 20 min of rewarming was maximal at 2-7 mM-K and yielded 15.5 mV. The hyperpolarization amounted to 7-2 and 10 mV at 0-4 and 10-8 mM-K, respectively. No hyperpolarization occurred in K-free solutions.