Bodyweights (BW) of 39 horses, competing in a 3-day event, were measured the evening before the endurance test (BW1), after phase D (BW2) and the next morning before stadium jumping (BW3). Plasma sodium concentration [Na1] was measured the evening before and after phase C of the endurance test [Na2]. Total body water (TBW1) was calculated using the formula (BW1 (kg) × 0.66 l/kg) and water loss (WL) was calculated as [(BW1 – BWx) X 0.9]. Cation balance (CB), the exchangeable cation concentration, was calculated using the formula [(TBW1(1) X Na1) - (TBW2(1)] X Na2). The decrease in BW and TBW averaged 17.4 ± 1.0 kg and 15.6 ± 0.8 litres respectively (mean ± s.e.m.) during the endurance test and remained decreased 9.2 ± 1.0 kg (BW) and 8.3 ± 1.0 litres (TBW) for 18 to 24 h after competition. Plasma sodium concentration after phase C of the endurance test (141.2 ± 0.3 mmol/l) was greater than resting values (138.8 ± 0.2 mmol/l), resulting in a net decrease in CB (-1601.1 ± 171.7 mmol). Significant water and electrolyte losses occur in horses competing in the endurance test of a 3-day event and these fluid losses persist for 18–24 h after this phase of competition.