Twenty Holsteins, ten pregnant heifers and ten pregnant cows (third or greater pregnancy) were subdivided and fed either a low calcium (Ca) or Ca-supplemented ration for four weeks prepartum to determine the influence of age and prepartum Ca intake on hormonal control of peripartum Ca homeostasis. Jugular blood samples were taken on a fixed schedule from 21 days prepartum through 21 days postpartum for parathyroid hormone (PTH), calcitonin (CT), Ca, magnesium (Mg), and phosphorus (P) analysis. Heifers and cows receiving the high Ca ration prepartum tended to have higher prepartum serum Ca. Cows fed the high Ca ration prepartum (hi-Ca cows) exhibited severe hypocalcemia (6.1 mg/dl) at parturition and remained hypocalcemic for three days. Serum PTH concentration increased prepartum (-5 to -3 days) and at parturition, followed by an increase in CT, in all groups except high-Ca cows. Circulating CT was lower in high-Ca cows throughout the experiment. Serum concentrations of PTH and Mg increased from 7 to 21 days in all except high-Ca cows. Feed intake corrected for metabolic bodyweight was similar for both dietary treatments and ages. Milk production was greater for the first week in cows fed low Ca prepartum. There was no correlation between hypocalcemia and increased milk Ca concentration. In conclusion, heifers were able to achieve calcium homeostasis despite the high Ca ration, while high-Ca cows exhibited subclinical milk fever. / M.S.