Dietary protein during conditioning and exercise must support additional needs while avoiding adverse metabolic effects. Ten Arabian horses were assigned randomly to 2 diets formulated to contain 7.5 or 14.5% crude protein and 12% fat. The low-protein diet (LP) was supplemented with lysine and threonine to match the levels of these amino acids in the high-protein diet (HP). Feed intake averaged 8.1 kg/day. Dietary cation-anion difference was 181.6 and 260.4 mmol/kg for high and low protein, respectively. Following 9 weeks conditioning, horses performed a repeated sprint test: 3 min walk at 1.5 m/s and zero slope, followed by 3 min walk at 1.5 m/s, 5 min trot at 3.5 m/s, then six 1 min sprints at 10 m/s separated by 4 min walks all on a 6% slope, concluding with 30 min walk at 1.5 m/s and zero slope. Blood samples (arterial, A, and venous, V) were taken at rest, during the last 15 s of each sprint, and at 5, 10, 20 and 30 min. of recovery. Samples were analysed for total protein (TP), lactate (La-), pH, PCO2, PO2, HCO3-, Na+, K+ and Cl-. Strong ion difference (SID+) and total weak acids (Atot) were calculated. Analysis of variance with repeated measures was used to evaluate the effects of exercise and recovery (time), diet and any interactions. Comparing LP to HP groups, plasma pH tended to be higher after the first sprint (V, P = 0.084; A, P = 0.014), and plasma HCO3- was higher overall (V, P = 0.0023; A, P = 0.094) during exercise and recovery. In both groups, pH declined; however, LP remained higher than HP. The decline in pH represents an exercise-induced acidosis which was attributable mainly to PCO2, especially during recovery, and to a tendency for a higher SID+ during most of exercise and recovery. The plasma La- response was lower, Cl- higher, suggesting that LP enhanced the lactate-storage function of erythrocytes. Dietary protein restriction for 9 weeks moderated sprint-induced acidosis in fat-adapted horses.