Abstract: Summary Four geldings and four mares of primarily Thoroughbred and Quarter Horse breeding were used in simultaneous 4x4 Latin square experiments to test the effects of dietary cation-anion balance (DCAB), defined as meq ((Na+K)-C1)/kg diet dry matter, on blood pH, blood gases, acid-base status and plasma glucose in horses at rest and following anaerobic exercise. Four diets, consisting of a base concentrate of corn, soybean meal and cottonseed hulls fed in a 60:40 ratio with Bermudagrass hay, were formulated to provide a DCAB of 5 (L=Low), 107 (ML=Medium Low), 201 (MH=Medium High) and 327 (H=High) meq ((Na+K)-C1)/kg diet dry matter. Calcium chloride and ammonium chloride were added to treatment L and ML and sodium bicarbonate and potassium citrate were added to treatment H to achieve the desired cation-anion balance. Treatment MH was not supplemented and served as the control treatment. Prior to the experiment, horses performed six weeks of long, slow, distance (LSD) work. During the experimental periods, horses were subjected to an exercise regimen alternating LSD with an interval training protocol 6 days/week. Venous blood pH, pCO2 and bicarbonate levels were significantly lower in horses at rest consuming diet L versus diets MH and H. Blood pH and acid-base parameters decreased with decreasing DCAB. There were no significant differences in blood pH or acid-base parameters between treatments, after anaerobic exercise. Plasma glucose concentrations for treatment L were significantly lower than treatment H at 10, 20, and 30 minutes post-exercise. These data suggest that DCAB has significant effect on acid-base status and indicate that horses consuming diets with low DCAB may experience a metabolic acidosis.