Summary An initial experiment (Experiment I) was conducted utilizing five mature Quarter Horses to establish baseline physiological responses to typical reining training. In an initial standardized exercise test (SET) which simulated reining horse maneuvers, heart rate and plasma lactate concentration indicated that galloping circles, spinning and stopping were anaerobic maneuvers (203 beats/min and 8.86 mmol/L, respectively). However, lactate concentrations declined before the end of the SET. The values were used to modify the SET to a degree of difficulty that would elicit significant anaerobiosis, thus maintaining elevated lactate concentrations throughout the SET. In a subsequent experiment (Experiment II), ten mature Quarter Horses were exercised by reining horse training in a crossover experiment. Horses were fed a control (C) and a 10% fat-supplemented (F) concentrate with bermuda grass hay in a 65:35 ratio. Heart rate (HR), respiration rate (RR), rectal temperature (RT) and venous blood samples were taken prior to, during and following recovery from a modified SET which simulated reining horse maneuvers but was more demanding than the previous SET. Heart rates and plasma lactate concentrations indicated that all maneuvers, except loping circles elicited anaerobiosis (208 beats/min and 11.8 mmol/L, respectively; peak values on d 0). Plasma glucose concentration fell while loping circles from resting concentrations of 104.3 mg/dl to 79.2 mg/dl increased throughout the remainder of the SET to 89.7 mg/dl and returned to resting concentrations by 30 min of recovery. Respiration rate, packed cell volume (PCV), rectal temperature and total serum non-esterified fatty acid concentration (NEFA) increased throughout the SET and peaked between the end of exercise and after 10 min of recovery (128 breaths/min; 51%; 39.9°C and .871 mEq/L, respectively). Diet composition had no consistent effects on physiological responses, but there were training effects. Heart rate and plasma lactate were lower on day 28 than on day 0 (P<.05) while plasma glucose, NEFA and PCV were not affected by training. Respiration rate and rectal temperature reflected ambient conditions.