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1Changes in the concentration of interstitial K+surrounding skeletal muscle fibres ([K+]I) probably play some role in the regulation of cardiovascular adjustments to muscular activity, as well as in the aetiology of muscle pain and fatigue during high‐intensity exercise. However, there is very little information on the response of [K+]Ito exercise in human skeletal muscle.2Five young healthy subjects performed plantar flexion exercise for four 5 min periods at increasing power outputs (≈1‐6 W) with 10 min intervening recovery periods, as well as for two 5 min periods with ischaemia at ≈1 and ≈3 W. Microdialysis probes were inserted into the gastrocnemius medialis muscle of the right leg to measure [K+]I, and K+release from the plantar flexors during and after incremental exercise was calculated from plasma flow and arterial‐venous differences for K+. Calf muscle pain was assessed using a visual analogue scale.3On average, [K+]Iwas 4.4 mmol l−1at rest and increased during minutes 3‐5 of incremental exercise by ≈1‐7 mmol l−1as a positive function of power output. K+release also increased as a function of exercise intensity, although there was a progressive increase by ≈1‐6 mmol l−1in the [K+] gradient between the interstitium and arterial‐venous plasma.4[K+]Iwas lower during ischaemic exercise than control exercise. In contrast to this effect of ischaemia on [K+]I, muscle pain was relatively higher during ischaemic exercise, which demonstrates that factors other than changes in [K+]Iare responsible for ischaemic muscle pain.5In conclusion, this study has demonstrated that during 5 min of dynamic exercise, [K+]Iincreases during the later period of exercise as a positive function of exercise intensity, ischaemia reduces [K+]Iduring rest and exercise, and the increase in [K+]Iis not responsible for muscle pain during ischaemic exercise.