This study was undertaken to investigate whether the mechanism of increased Na+–K+–2Cl (NKCC1) cotransporter activity by osmotic shrinkage involved AMP-activated protein kinase (AMPK) activation. AMPK was found to phosphorylate a recombinant GST-dogfish (1–260) NKCC1 fragment at Ser38 and Ser214, corresponding to Ser77 and Ser242 in human NKCC1, respectively. Incubation of human erythrocytes with 20 μm A769662 AMPK activator increased Ser242 NKCC1 phosphorylation but did not stimulate 86Rb+ uptake. Under hypertonic conditions in human red blood cells (RBCs) incubated with 0.3 m sucrose, NKCC1 activity increased as measured by bumetanide-sensitive 86Rb+ uptake and AMPK was activated. However, there was no effect of AMPKα1 deletion in mouse RBCs on the increased rate of 86Rb+ uptake induced by hyperosmolarity. AMPK activation by osmotic shrinkage of mouse RBCs was abrogated by 10 μm STO-609 CaMKKβ inhibitor, but incubation with STO-609 did not affect the increase in 86Rb+ uptake induced by hyperosmolarity. Osmotic shrinkage of human and mouse RBCs led to activation loop phosphorylation of the STE20/SPS1-related proline/alanine-rich kinase (SPAK) at Thr233, which was accompanied by phosphorylation of NKCC1 at Thr203/207/212, one of which (Thr207) is responsible for cotransporter activation. Therefore, phosphorylation-induced activation of NKCC1 by osmotic shrinkage does not involve AMPK and is likely to be due to SPAK activation.