EHDP is a synthetic analogue of pyrophosphate and a potent inhibitor of bone resorption. The objective of this investigation was to evaluate the effects of EHDP on the ultrastructure and function of parathyroid glands stimulated by feeding a low calcium diet to pregnant cows.Two groups of 3 pregnant cows each were fed a low calcium (5 gm/day) diet supplemented with phosphorus (25 gm/day) for approximately 70 days prepartum and up to 2 days postpartum. Cows of one group received a daily subcutaneous injection of EHDP (4 mg/kg/day). Cows of the second group received no EHDP. At parturition cows fed the low calcium diet remained clinically normal and had serum calcium and phosphorus values of 8.05 and 4.3 mg/100 ml, respectively (Fig. 1). EHDP-treated cows developed hypocalcemia (5.44 mg/100 ml), hypophosphatemia (1.49 mg/100 ml), and clinical signs pathogonomic for parturient paresis. Plasma immunoreactive parathyroid hormone (iPTH) levels were as high or higher both prepartum and postpartum than in cows only fed the low calcium diet.