Purpose: The cardiac T-wave peak-to-end interval (T_pe) is thought to reflect dispersion in ventricular repolarisation, with abnormalities in T_pe associated with increased risk of arrhythmia. Extracellular K⁺ modulates cardiac repolarisation, and since arterial plasma K⁺ concentration ([K⁺]) rapidly increases during and declines following exercise, we investigated the relationship between [K⁺] and T_pe with exercise. Methods: Serial ECGs (T_pe, T_pe/QT ratio) and [K⁺] were obtained from 8 healthy, normokalaemic volunteers and 22 patients with end-stage renal disease (ESRD), at rest, during, and after exhaustive exercise. Results: Post-exercise [K⁺] nadir was 3.1 ± 0.1, 5.0 ± 0.2 and 4.0 ± 0.1 mmol.L⁻¹ (mean ± SEM) for healthy participants and ESRD patients before and after haemodialysis, respectively. In healthy participants, compared to pre-exercise, recovery-induced low [K⁺] was associated with a prolongation of T_pe (110 ± 8 vs. 87 ± 5 ms, respectively, p = 0.03) and an increase in T_pe/QT ratio (0.28 ± 0.01 vs. 0.23 ± 0.01, respectively, p = 0.01). Analyses of serial data revealed [K⁺] as a predictor of T_pe in healthy participants (β = -0.54 ±0.05, p < 0.0001), in ESRD patients (β = -0.75 ± 0.06, p < 0.0001) and for all data pooled (β = -0.61 ± 0.04, p < 0.0001). The [K⁺] was also a predictor of T_pe/QT ratio in healthy participants and ESRD patients. Conclusions: T_pe and T_pe/QT ratio are predicted by [K⁺] during exercise. Low [K⁺] during recovery from exercise was associated with increased T_pe and T_pe/QT, indicating accentuated dispersion of ventricular repolarisation. The findings suggest that variations in [K⁺] with physical exertion may unmask electrophysiological vulnerabilities to arrhythmia.