The early plasticity of vastus lateralis Na⁺-K⁺-ATPase to the abrupt onset of prolonged submaximal cycling was studied in 12 untrained participants (V0_{2 peak} =44.8±2.0 ml.kg^-1.min^-1, 0±SE) using a 6 day protocol (3 days of exercise plus 3 days of recovery). Tissue samples were extracted prior to (Pre) and following exercise (Post) on day 1 (E1) and day 3 (E3) and on each day of recovery (R1, R2, R3) and analyzed for changes in maximal protein (max) (vanadate-facilitated [³H] ouabain binding), and isoform concentration (quantitative immunoblotting) and maximal Na⁺-K⁺-ATPase activity (V_max) (3-0-methylfluorescein K⁺-stimulated phosphatase assay). For max (pmol.g-1wet wt), an increase (P0.05) of 11.8% was observed at R1 compared to E1-Pre (340±14 vs 304±17). For the isoforms, ₁, ₂, ₃, increases (P0.05) of 46%, 42% and 31% were observed at R1, respectively. For the isoform, ₁ and ₂ increased (P0.05) by 19% and 28% at R1 while ₃ increased (P0.05) by 18% at R2. With the exception of α2 and α3, the increases in the isoforms persisted at R3. Exercise resulted in an average decrease (P0.05) in V_max by 14.3%. No differences were observed in V_max at E1 — Pre and E3 — Pre or between R1, R2 and R3. It is concluded that 3 days of prolonged exercise is a powerful stimulus for the rapid upregulation of the Na^+-K+-ATPase subunit isoforms. Contrary to our hypothesis, the increase in subunit expression is not accompanied by increases in the maximal catalytic activity.