Ten pregnant heifers and 10 pregnant cows (three or more pregnancies) were assigned to groups of five and fed either the recommended (low) amount of calcium or excess (high) calcium in their diet for 4 wk before parturition to determine the influence of prepartum calcium intake and age on hormonal control of peripartum calcium homeostasis. After parturition all groups received a diet with calcium adequate for lactation. Jugular blood samples from 21, 14, 7, 6, 5, 4, 3, 2, and 1 d prepartum through 0, 1, 2, 3, 5, 7, 14 and 21 d postpartum were assayed for concentration of parathyroid hormone, calcitonin, calcium, magnesium and phosphorus. Heifers and cows receiving high calcium diets had higher calcium and lower parathyroid hormone in blood serum before parturition than animals receiving the low calcium diets. Cows, but not heifers, fed high calcium diets exhibited severe hypocalcemia at parturition, remained hypocalcemic for 3 d and had low serum calcitonin. Regardless of dietary group, concentrations of parathyroid hormone and magnesium in serum increased after the first week of lactation. Feed intake during lactation, corrected for metabolic body weight, was similar for both dietary treatments and ages. Milk production per kilogram metabolic body weight was highest during the first week of lactation for cows fed low calcium diets before parturition. There was no correlation between hypocalcemia and loss of calcium in colostrum or milk. Feeding low dietary calcium to cows in the prepartum period was effective in the prevention of severe hypocalcemia at parturition. In contrast, dietary treatment of heifers had no effect on serum calcium concentration at parturition.(ABSTRACT TRUNCATED AT 250 WORDS)