In milk fever (parturient paresis), calcium homeostatic mechanisms, regulated by parathyroid hormone and 1,25-dihydroxyvitamin D, fail to maintain normal blood calcium concentrations, resulting in severe hypocalcemia. The precise nature of the endocrine defect is unknown. Secretion of parathyroid hormone and production of 1,25-dihydroxyvitamin D is similar in most cows with milk fever or without. However, there are some cows that fail to produce adequate 1,25-dihydroxyvitamin D at the onset of lactation. These tend to be cows that will suffer prolonged hypocalcemia and relapse after treatment. Assuming that most cows produce adequate amounts of both hormones, the next logical cause of milk fever might be a failure of tissues to respond to calcium-regulating hormones. Older cows are more likely to develop milk fever than younger ones. We have found that tissue 1,25-dihydroxyvitamin D receptor concentrations decline with age, leaving the tissues less able to respond to 1,25-dihydroxyvitamin D. We also have found that tissue 1,25-dihydroxyvitamin D receptor concentrations increase during pregnancy and lactation in the cow. Intestinal 1,25-dihydroxyvitamin D receptor concentration does not appear to be different in cows with or without milk fever in cows of similar ages. However, intestinal 1,25-dihydroxyvitamin D receptor numbers decrease precipitously at parturition, which may in part be responsible for the development of hypocalcemia in dairy cattle.