Tumor cell undergo a rapid change in their K ion content as they are incubated several degrees above the physiological temperature. The K content of P-815 mouse mastocytoma cells decreases while efflux of the ions increases. The ouabain sensitive pumping rate for $^{86}Rb$ suffers a reduction of 40% within an hour. On the other hand, the mouse red blood cells do not indicate any change in the rate of $^{86}Rb$ influx over a 60 min incubation at $43^{circ}C$. The change in the Na content is not detectable either by the flame emission or by isotope counting. The Na influx, however, increases somewhat soon after the temperature is shifted to $43^{circ}C$. Since the K content decreases much more than the Na content increases, the net cation content in the cells decrease. This ionic imbalance should result in negatively charged cells unless the culls also lose anions or accumulate other canons.