During the transition period (3 wk pre- to 3 wk post-calving), there is a rapid increase in the requirement for calcium (Ca). This Ca is deposited into the fetus before birth, and at the start of lactation is secreted in large amounts into colostrum. The sudden demand for the secretion of large amounts of calcium rapidly decreases the blood calcium levels of the cow. When this decrease is not corrected for by homeostatic mechanisms, a disorder known as hypocalcemia, or milk fever, occurs. While the causes for this failure of homeostatic mechanisms can vary, the effects of this metabolic disorder are equally harmful. Hypocalcemia predisposes to many other problems, such as ketosis, decreased milk yield, decreased reproduction, and increased incidence of mastitis. While treatment of hypocalcemia is possible, it is more beneficial to prevent the disorder before onset. Recently, it has been discovered that serotonin (5-HT) plays a role in regulating calcium concentrations in the mammary gland. As it is understood, parathyroid hormone related protein (PTHrP) secreted within the mammary epithelial cells (MECs) is responsible for the mobilization of calcium from bones to enter the blood stream at the onset of lactation, and the secretion of PTHrP is induced by increased 5-HT levels in the blood. Therefore, studies have been performed to determine whether or not the presence of 5-HT does in fact better regulate calcium homeostasis. Through both rodent studies and studies with lactating dairy cows, it has been determined that pre-calving treatment with 5-hydroxy-L-tryptophan, a 5-HT precursor, (or 5-HTP) raises serum 5-HT levels and PTHrP levels in the mammary tissue, and that circulating Ca levels are then decreased. This decrease in circulating Ca induces Ca mobilization from bone. This early mobilization of Ca prepares for the sudden increased demand at the onset of lactation, and allows for better maintenance of maternal Ca homeostasis. Because of these conclusions, it has been determined that treatment of cows with 5-HTP before calving is a potential method for the prevention of hypocalcemia.