Twenty-one Ayshire cows were randomly assigned to one of three diets to determine the effect of an anionic diet and high potassium (K) intake on mineral metabolism, acid-base status and feed intake of dairy cows fed grass silage based diets during the dry period. Dietary cation-anion balance (DCAB), calculated as milliequivalents [(Na⁺ + K⁺ ) - (CI^- + S^2-)], for high DCAB (control), high DCAB + K-supplement (added as KHCO₃) and low DCAB treatments were +298, +571 and +107 mEq/kg DM, respectively. Dietary magnesium (Mg) content, equivalent to a daily Mg intake of 33 g, was on average 0.4% (in a DM basis). Cows received grass silage (5.2 kg DM), hay (0.9 kg DM) and concentrate mixture (2.7 kg DM) until calving. Blood and urine samples were collected 4, 3, 2 and 1 week before the expected calving date, at calving, the day and 1 week after calving. Only urinary pH was significantly affected by a low DCAB prepartum. K supplementation decreased fractional excretion of Mg and Na in the urine and significantly increased prepartum urinary K excretion. A dietary K concentration of 34 g/kg DM coupled with a high Mg intake of 4 g/kg DM in the prepartum diet may negatively effect Mg metabolism after parturition.