Dietary cation-anion difference (DCAD) affects animal performance (Tucker, 1988). To understand mechanisms involved, two groups of lactating fistulated cows received one of two concentrate levels (20 or 40% DM). Each group were assigned to two 3×3 Latin squares to test three DCAD (0, 150 and 300 mEq/kg DM) on performances, ruminal fluid and acid-base status. Feed intake and fat corrected milk yield were enhanced by increasing DCAD. Responses were greater with the high concentrate diet. Rumen pH and VFA were slightly affected by treatments while acid-base status was very responsive, suggesting that increasing DCAD improves performance through changes in acid-base status.