The present study tested the hypothesis that exercise with a large compared with a small active muscle mass results in a higher contraction-induced increase in Na⁺-K⁺-ATPase mRNA expression due to greater hormonal responses. Furthermore, the relative abundance of Na⁺-K⁺-ATPase subunit a₁, a₂, a₃, a₄, ^ß₁, ^ß₂, and ^ß₃ mRNA in human skeletal muscle was investigated. On two occasions, eight subjects performed one-legged knee extension exercise (L) or combined one-legged knee extension and bilateral arm cranking (AL) for 5.00, 4.25, 3.50, 2.75, and 2.00 min separated by 3 min of rest. Leg exercise power output was the same in AL and L, but heart rate at the end of each exercise interval was higher in AL compared with L. One minute after exercise, arm venous blood lactate was higher in AL than in L. A higher level of blood epinephrine and norepinephrine was evident 3 min after exercise in AL compared with L. Nevertheless, none of the exercise-induced increases in a₁, a₂, ^ß₁, and ^ß₃ mRNA expression levels were higher in AL compared with L. The most abundant Na⁺-K⁺-ATPase subunit at the mRNA level was ^ß₁, which was expressed 3.4 times than a₂. Expression of a₁, ^ß₂, and ^ß₃ was less than 5% of the a₂ expression, and no reliable detection of a₃ and a₄ was possible. In conclusion, activation of additional muscle mass does not result in a higher exercise-induced increase in Na⁺-K⁺-ATPase subunit-specific mRNA.