The effects of feeding diets with different milliequivalents (mEq) of dietary ([Na+ + K+]-[Cl- + SO4 =]) to dairy cows during the last seven weeks of pregnancy on bone morphology at parturition were studied. Nine monozygotic twin pairs of pregnant cows (five pairs of parity 1 or 2 and four pairs of parity 3 or more) were allocated to two diets which were formulated to provide either -4 mEq (anion diet) or +572.5 mEq (cation diet) of ([Na+ + K+]-[Cl- + SO4 =]) kg-1 dietary dry matter. Bone biopsies were taken from the tuber coxae between three and eight hours after parturition. The plasma concentrations of calcium and inorganic phosphorus, the total plasma alkaline phosphatase activity and the urinary hydroxyproline:creatinine ratio were not significantly affected by diet during the experimental period. In low parity (2 or less) cows the percentage trabecular bone volume, the percentage osteoclast surface and the mean number of osteoclasts per microscopic field (identified by Goldner staining) were lower on the anion diet than on the cation diet (P < 0.02). In the high parity cows, the percentage osteoid volume (P < 0.05) and the ratio of percentage osteoid volume to percentage osteoid surface (P < 0.001) were greater in the cows fed the anion diet than in the cows fed the cation diet. The results show that reducing the mEq of dietary ([Na+ + K+]-[Cl- + SO4 =]) to -4 mEq kg-1 dietary dry matter affected some of the parameters of bone formation but did not enhance bone resorption.