The hypothesis was tested that exercise-induced changes in plasma composition stimulate unidirectional K+ (JinK) and net K+ (JnetK) transport in human red blood cells (RBCs). Experiment 1 (7 males, 2 females), used an exercise simulated (ES) plasma (resting plasma manipulated to mimic exercise plasma by increasing osmolality, [K+], [H +], [lactate] and [epinephrine] to exercise levels) to incubate resting RBCs and a K+ selective electrode to continuously measure JnetK. Experiment 2 (7 males, 4 females), utilized true exercise plasma (TEP) (obtained during a 30 s high intensity leg cycling exercise) to determine JinK via RBC 86Rb uptake and JnetK. The JnetK of Resting RBCs in experiment 1 was significantly elevated from 0.9 ± 28.7 meq · L RBCs -1 · hr-1 in resting plasma to 384.8 ± 164.3 meq · L RBCs-1 · hr -1 in ES plasma and in experiment 2 from -5.79 ± 16.09 meq · L RBCs-1 · hr-1 to 177.87 ± 59.87 meq · L RBCs-1 · hr-1 at 10s of incubation in TEP. JinK for resting RBCs in TEP was significantly elevated from control to 29 899 ± 8 680 [mu]mol · L RBCs-1 · hr-1 at 10 s. Peak JinK and JnetK occurred at the onset of incubation due to the Na +-K+ pump, Na+-K+-2Cl - cotransporter, and diffusion. It is concluded RBCs increased K+ transport in response to exercise-induced changes in plasma composition.