Accumulation of K+ in skeletal muscle interstitium during intense exercise has been suggested to cause fatigue in humans. The present study examined interstitial K+ kinetics and fatigue during repeated, intense, exhaustive exercise in human skeletal muscle. Ten subjects performed three repeated, intense (61.64.1W; meanSEM), one-legged knee extension exercise bouts (EX1, EX2 and EX3) to exhaustion separated by 10-min recovery periods. Interstitial [K+] ([K+]interst) in the vastus lateralis muscle were determined using microdialysis. Time-to-fatigue decreased progressively (P0.05) during the protocol (5.10.4, 4.20.3 and 3.20.2min for EX1, EX2 and EX3 respectively). Prior to these bouts, [K+]interst was 4.10.2, 4.80.2 and 5.20.2mM, respectively. During the initial 1.5min of exercise the accumulation rate of interstitial K+ was 85% greater (P0.05) in EX1 than in EX3. At exhaustion [K+]interst was 11.40.8mM in EX1, which was not different from that in EX2 (10.40.8mM), but higher (P0.05) than in EX3 (9.10.3mM). The study demonstrated that the rate of accumulation of K+ in the muscle interstitium declines during intense repetitive exercise. Furthermore, whilst [K+]interst at exhaustion reached levels high enough to impair performance, the concentration decreased with repeated exercise, suggesting that accumulation of interstitial K+ per se does not cause fatigue when intense exercise is repeated.