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1. The effect of various intracellular Na concentrations ([Na]_i) on the membrane potential after hypothermia was studied in guinea-pig auricles.

2. For varying [Na]_i, the atria were cooled for 4 hr at 4-6° C in a K-poor solution with different concentrations of NaCl. The auricles were rewarmed in normal Tyrode solution at 35° C.

3. Extracellular space (ECS), intracellular Na and K concentrations ([Na]_i and [K]_i) and membrane potential of the atria were measured before and after hypothermia.

4. The ECS, measured as inulin space, amounted to 350 ml./kg wet wt. at 35° C and to 300 ml./kg wet wt. at 4-6° C.

5. [K]_i decreased during cooling and increased during rewarming the auricles. [Na]_i increased during hypothermia in bathing fluids containing NaCl, but decreased in NaCl- and Na-free solutions. At the beginning of rewarming a net Na transport occurred from cells with high [Na]_i, while a net Na uptake took place in atria with low [Na]_i.

6. At the same time, the membrane potential of auricles with increased [Na]_i hyperpolarized beyond the steady-state value recorded at the end of rewarming, or even beyond the calculated K⁺ equilibrium potential (E_K). Afterwards, the hyperpolarization levelled off, while the E_K values increased further. The membrane potential of atria with decreased [Na]_i showed no transitory hyperpolarization during rewarming.

7. The hyperpolarization beyond the steady-state value of membrane potential in rewarmed auricles was significantly correlated to the active Na efflux.

8. From these results it is concluded that the membrane potential of guinea-pig atria after hypothermia is affected by an active, electrogenic Na pump activated by intracellular Na ions.