Eleven Standardbred horses were tested to assess plasma electrolyte fluctuations during different intensities of exercise (at rest, immediately following 20 min of jogging, and immediately following a 2 min race), with and without an orally administered electrolyte supplement 2 h prior to sample collection. Exercise treatments were repeated after horses were given an oral dosage of an electrolyte supplement 2 h prior to sample collection. Data were analyzed using a repeated incomplete 6 × 6 Latin-square design with a 3 × 2 factorial arrangement of treatments (exercise × supplement). Jogging horses had an elevated heart rate as compared with resting horses and plasma potassium concentrations that were higher than those of the other two treatments (P < 0.05). Racing horses had the highest heart rate of the three treatments and plasma sodium concentrations that were higher than those of resting and jogging horses (P < 0.05). Provision of an electrolyte supplement significantly increased heart rate, as well as blood potassium and sodium concentrations. Further studies on the dietary electrolyte supplementation would benefit from investigating additional performance and recovery parameters, with focus on the hydration status of the horse before, during, and after exercises.