The hypothesis was tested that exercise-induced changes in plasma composition stimulate unidirectional K⁺ transport (J_inK) in human red blood cells (RBCs). Ten men performed two 30-s high-intensity leg-cycling tests separated by 4 min of rest. Antecubital venous blood was sampled before exercise and at the end of the second exercise bout. RBCs were separated from true exercise plasma, ⁴²K was added to plasma, and RBC K⁺ transport was studied in vitro at 37°C. In the second part of the study, blood from nine healthy men studied in vitro at 37°C was used to test the hypothesis that exercise-simulated (ES) plasma stimulates net K⁺ transport and J_inK (measured using ⁸⁶Rb) in human RBCs. The J_inK of resting RBCs added to true exercise plasma was 1,574 ± 200 (SE) µmol · h¹ · l¹ vs. 1,236 ± 256 µmol · h¹ · l¹ in true resting plasma at 2 min (controls). In true exercise and ES plasma, J_inK was increased through activation of the ouabain-sensitive Na⁺-K⁺ pump and the bumetanide-sensitive Na⁺-K⁺-2Cl cotransporter. Increases in plasma osmolality and K⁺, H⁺, and epinephrine concentrations independently and in combination stimulated K⁺ transport into human RBCs. In a third series of experiments, in which ES plasma K⁺ concentration was continuously measured during the first 5 min of incubation of RBCs, a 1.6 ± 0.3 mmol/l decrease in plasma K⁺ concentration occurred during the first 2 min. It is concluded that RBCs transport K⁺ at elevated rates in response to exercise-induced changes in plasma composition.