Na+‐K+‐2Cl− cotransport mediates the movement of two Cl− ions for one Na+ and one K+ ion. Under isosmotic conditions or SPAK/WNK4 activation, the NKCC1‐mediated bumetanide‐sensitive Cl− uptake in Xenopus laevis oocytes, as measured using 36Cl, is twice the value of K+ uptake, as determined using 86Rb. In contrast, under hyperosmotic conditions, there is a very significant activation of the bumetanide‐sensitive K+ uptake with only a minimal increase in Cl− uptake. This indicates that when stimulated by hypertonicity, the cotransporter mediates Na+‐K+‐2Cl− cotransport as well as K+/K+ exchange. Interestingly, this stimulation of K+/K+ exchange was only observed with NKCC1, but not NKCC2. In order to identify the molecular determinant(s) of this NKCC1‐specific activation, we created chimeras of the mouse NKCC1 and the rat NKCC2. Swapping the regulatory amino termini of the cotransporters neither conferred activation to NKCC2, nor prevented activation of NKCC1. Using unique restrictions sites, we created additional chimeric molecules and determined that the second extracellular loop between membrane spanning domains three and four is involved in the hyperosmotic stimulation of K+ uptake (K+/K+ exchange) in NKCC1. However, replacing only this loop in NKCC2 was insufficient for K+ uptake activation, suggesting an interaction between this loop and a yet to be identified region of the cotransporter.