To examine the effects of balloon mitral valvotomy (BMV) on exercise-induced hyperkalemia, and on changes in the concentration of Na,K-pumps in skeletal muscle, we evaluated eight subjects with mitral stenosis before BMV, and at two weeks and four months after BMV. Subjects underwent incremental exercise to exhaustion for exercise-induced rise in [K⁺] and vastus lateralis muscle biopsy for concentration of Na,K-pumps. Mean (± SE) valve area increased from 0.89 [pm] 0.03 cm² before to 1.75 ± 0.05 cm² after BMV. The rise in [K⁺] with absolute workload fell progressively at early and late follow up post-BMV (p <0.05). The concentration of Na,K-pumps was similar to baseline at early follow up (233 ± 10 versus 228 ± 15 pmol/g wet weight), but was significantly increased at late follow up (265 ± 17 pmol/g; p <0.05). There was a negative correlation between the concentration of Na,K-pumps and the exercise-induced rise in [K⁺]. However, correlation analysis for the effects of changes in Na,K-pumps on changes in exercise hyperkalemia after BMV was not significant. The progressive reduction in exercise-induced rise in [K⁺] after BMV may contribute to the progressive improvement in exercise performance. The increased concentration of Na,K-pumps in skeletal muscle may contribute to this improvement, and emphasizes the importance of peripheral adaptations in clinical improvement after BMV.