The effects of hemorrhagic shock and whole blood transfusions on red blood cell (RBC) Na+, K+, and ATP levels have been studied using two shock models. Group A animals were bled to a pressure of 40 mm Hg, following which no further blood was removed or returned. The blood pressure of these rats increased to approximately 70 mm Hg within 30 min and remained at that level for 2 hr. Group B animals were bled to a pressure of 40 mm Hg and maintained at that level for 2 hr. No significant changes in RBC Na+ or K+ levels were found in Group A animals subjected to shock. However, small but significant changes in RBC cations were observed in Group B animals subjected to hemorrhagic shock. In Group B, ATP levels decreased from 1.51 to 1.27 mmole/liter RBC. In another study (Group C), rats were subjected to shock as in Group B but were transfused with rat's whole blood which had been stored in cold ACD buffer for 6 days instead of their own shed blood. RBC Na+ of Group C increased from 3.5 ± 0.1 to 8.4 ± 0.7, and K+ decreased from 100.3 ± 2.3 to 85.0 ± 2.1. The RBC Na+ of stored blood itself increased by 288 and 355% at 3 and 6 days cold storage, respectively. These results indicate that (1) severe hemorrhagic shock produces small changes in RBC Na+ and K+ levels; and (2) large changes in RBC cation levels which have been reported previously in man may be due to the additional effect of transfusion of stored blood. © 1981.