PURPOSE: Sweat-induced fluid loss during prolonged exercise-heat stress can compromise cardiovascular and thermoregulatory function, although its effects on cardiac autonomic modulation remain unclear. We therefore examined heart rate variability (HRV) and recovery (HRRec), as surrogates of cardiac autonomic modulation, during and following prolonged exercise in the heat with and without fluid replacement. METHODS: Eleven young and healthy men performed 90-min of semi-recumbent cycling in dry-heat (40 degrees C; 20% relative humidity) at a fixed rate of metabolic heat production (600 W; ~46% V[Combining Dot Above]O2peak) followed by 40-min resting recovery without fluid replacement (No-FR; ~3.4% reduction in body mass). On a separate day, participants completed the same protocol with fluid replacement (FR; 500-700 ml timed boluses) to offset sweat losses. Esophageal temperature and electrocardiogram were recorded throughout, with measurements analyzed over 10-min averaged epochs during baseline, each 30-min interval during exercise and 20-min interval during recovery. RESULTS: Esophageal temperature and heart rate was elevated in No-FR relative to FR throughout exercise (all P