To replace electrolytes lost in sweat during endurance competitions, riders frequently supplement horses with hypertonic oral electrolyte pastes. To determine whether this practice and concurrent administration of the so-called hyperhydrating substance glycerol are of benefit, weight loss, voluntary water intake, plasma osmolality, and plasma protein and electrolyte concentrations were measured in 6 Arabian horses 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) before and during a treadmill exercise test simulating a 60 km endurance ride. Weight loss was greater (P < 0.01) with W (3.2%) than with E and GE (1.0% and 0.9%, respectively) and was associated (r = -0.85, P < 0.0001) with less (P < 0.01) water intake with W (12.2 l) than with E or GE (23.5 l and 25.8 l, respectively). Plasma osmolality increased to a greater extent (P < 0.01) with GE than with E and was unchanged with W. In contrast, plasma protein concentration decreased (P < 0.01) in the later stages of the simulated ride with E and GE, reflecting plasma volume expansion, but remained unchanged with W. Plasma Na+ and Cl- concentrations increased (P < 0.01) with E and GE and were greater (P < 0.01) than values for W during the second half of the 60 km simulated ride. Despite administration are large amounts of potassium with E and GE, plasma K+ concentration was decreased (P < 0.01) at the end of the rest breaks during the simulated ride and after 60 min of recovery with all supplements. Adverse effects of administration of hypertonic oral electrolyte pastes were not observed and decreases in plasma protein concentration within 30 min after drinking with all supplements suggested that intestinal absorption was well maintained during the simulated endurance ride. In conclusion, electrolyte supplementation in the form of hypertonic oral pastes improved water intake during endurance exercise but concurrent glycerol administration provided no additional benefits in comparison to supplementation with electrolytes alone.