Incomplete recovery from endurance exercise after an overnight rest period is reflected by persisting weight loss and an elevated plasma aldosterone concentration, even in successful competitors. To determine whether supplementation with high doses of electrolytes, with or without glycerol, enhances recovery, the following were measured in 6 Arabian horses before and after completion of a 60 km treadmill exercise test simulating an endurance ride and after 12, 24, 48, and 72 h of recovery: bodyweight; plasma osmolality; plasma concentrations of protein, electrolytes, aldosterone and cortisol; and urine and faecal electrolyte concentrations. Before and during the exercise test, horses were supplemented with a total of 2.4 ml/kg bwt of water (W); 0.2 g/kg bwt KCl and 0.4 g/kg bwt NaCl in 2.4 ml/kg bwt of water (E); or 0.2 g/kg bwt KCl and 0.4 g/kg bwt NaCl in 2.4 ml/kg bwt (3 g/kg bwt) of glycerol (GE). Although weight loss after completion of the simulated ride was greater (P < 0.01) for W (3.2%) than for E and GE (1.0 and 0.9%, respectively), horses supplemented with E or GE experienced further weight loss by 24 h after the simulated ride (2.2 and 2.1% for E and GE, respectively) while bodyweight with W remained unchanged (3.0%) from the finish value. After 48 h of recovery, bodyweight was not different from the starting values with E and GE but remained decreased (P < 0.01) with W throughout the recovery period (2.2% persisting weight loss after 72 h of recovery). Plasma osmolality and plasma Na+ and Cl- concentrations increased (P < 0.01) and plasma protein concentration decreased (P < 0.01) after the exercise test with E and GE but were unchanged with W. Plasma osmolality and protein and electrolyte concentrations returned to pre-exercise values within 12 h of recovery with the exception of a persistent increase in plasma Na+ concentration with GE. The greatest plasma aldosterone concentration was measured after 12 h of recovery with W (1357 pmol/l) and was greater (P < 0.02) than that with E and GE (24 and 304 pmol/l, respectively). Urine production during the simulated ride increased (P < 0.01) with GE and resulted in loss of approximately 20% and essentially 100% of supplemented Na+ and K+, respectively. In contrast, electrolyte losses in faeces were unaffected by electrolyte or glycerol supplementation. In conclusion, supplementation with high doses of electrolytes as hypertonic oral pastes attenuated weight loss during a simulated endurance ride (by enhancing voluntary water intake); however, it did not prevent development of significant weight loss during the initial 24 h of recovery. Glycerol administration resulted in no benefits, and actually increased urine electrolyte losses, in comparison to supplementation with electrolytes alone.