Nations intending to send teams to compete in the equestrian sports at the 2008 Olympic Games in August in Hong Kong were concerned that safe competition would not be possible in this environment. Our objective was to demonstrate that horses could safely compete in the Hong Kong climate, where identifying and treating dehydration and electrolyte losses would be especially important to horse welfare. We hypothesized that evaluating serial changes in body weight, urine specific gravity (USG), urine Na concentration (UNa) and urinary clearance of Na (CLNa) would allow us to identify dehydration and Na deficits. Seventeen horses were housed in air-conditioned stables maintained at 25°C. Vital signs and body weight were recorded twice daily. Twenty-four-hour water consumption, and number and duration of exercise periods, were recorded daily. Haematological and biochemical variables, USG, UNa and CLNa, were measured intermittently in subsets of horses. Environmental conditions were monitored daily. Fifteen horses completed the competition; vital signs and all mean values for haematological and biochemical variables (except a high mean total protein on day 1 after arrival) remained within reference ranges. Daily water intake was (mean ± SE) 38 ± 1.5 l (range 18–74 l). Mean change in body weight during the entire study was − 3 ± 2.4 kg (range − 17 to 14 kg). Horses spent at most 3.5 h outside and at least 20.5 h in the stables daily. The mean value for USG was 1.035 ± 0.0008 and 57% of determinations were >1.035. The mean UNa was 61 ± 8 mmol l− 1 and 57% of values were ≤ 20 mmol l− 1. The mean CLNa was 0.14 ± 0.0.04%, with 76% of determinations being ≤ 0.1%. An 88% completion rate demonstrated that safe competition was possible. Access to climate-controlled stables probably helped to minimize daily fluid and electrolyte losses, and facilitated recovery following exercise. A majority of USG values >1.035, UNa < 20 mmol l− 1 in 29% of determinations, and CLNa < 0.1% in 57% of determinations suggested that some horses may have had insufficient Na intake while in Hong Kong. Monitoring USG, UNa and CLNa may aid in identifying and treating fluid and Na deficits in competition horses.