The dietary supplement ephedra is a potent sympathomimetic that was banned by the US Food and Drug Administration in 2003 because of its deleterious effects on cardiovascular function and thermoregulation during exercise. Unfortunately, extracts of ephedra can still be obtained via the internet and are in use worldwide. The horse is the only athletic species other than humans that sweats to thermoregulate and it controls cardiovascular function in a similar fashion. The purpose of this study was to use the horse to examine the acute effects of ephedra (Ma Huang) to investigate on markers of performance as well as effects on cardiovascular function and thermoregulation during acute exercise. Six Standardbred mares (~450 kg; 6-12 years of age) were used in a crossover design consisting of a ephedra (Ma Huang containing 8% ephedra alkaloid) and control (applesauce) group. All horses performed an incremental graded exercise test (GXT) at a 6% fixed grade to measure oxygen uptake (VO₂), run time, velocity at VO_2max, maximal velocity, recovery time, haematocrit, total plasma protein concentration, heart rate, right ventricular pressure (RVP), pulmonary arterial pressure (PAP), rectal temperature and recovery. Measurements were recorded at rest, during exercise and post 2 and 5 min recovery. There was a difference (P<0.05) in pre-exercise haematocrit but not in any other haematocrit or plasma protein sampling intervals. VO₂ was greater (P<0.05) for the ephedra group before exercise, at each step of the GXT, at VO_2max, and during recovery compared to the control group. Recovery time was significantly different, but run time was not (P≯0.05). Heart rate was elevated (P<0.05) at 2 and 5 min recovery in horses administered ephedra. Significant differences were observed for RVP and PAP and rectal temperature during recovery. Recovery score (sweating response, respiration rate, behaviour) was altered (P<0.05) by ephedra administration. These data suggests an increase in energy expenditure and thermogenesis when horses consume ephedra. However, markers of performance (run time, velocity at VO_2max, and maximal velocity completed) were not altered by ephedra administration.