Serotonergic regulation of calcium (Ca) metabolism during the peripartal period has only recently been elucidated. The discovery of mammary gland-derived serotonin and its subsequent interaction with peripartal Ca is a relatively new concept, particularly in the bovine model. Much progress has been made understanding serotonin’s role in regulating Ca status by utilizing the precursor to serotonin synthesis, 5-hydroxy-L-tryptophan (5HTP). A study performed in late-lactation dairy cows demonstrated that treatment with 5HTP decreased circulating total Ca concentrations and urine Ca concentrations, but increased milk Ca concentrations. This suggested that serotonin was potentially coordinating Ca flux between kidney, bone, gut, and milk. When peripartal multiparous cows were treated with 5HTP prepartum, total Ca concentrations increased postpartum compared with controls, and further improved when 5HTP was given in combination with a negative DCAD diet. Interestingly, cows treated with 5HTP had decreased circulating parathyroid hormone concentrations compared with controls and increased concentrations of urinary deoxypyridinoline, a bone resorption marker. Previous data in mice suggested serotonin induced transcriptional and translational changes in Ca transporters and pumps in the mammary gland. We confirmed these findings in the dairy cow, demonstrating that transcription of several pumps and transporters, along with parathyroid hormone related-protein were increased on d 8 of lactation in mammary glands of cows treated with 5HTP prepartum. In an attempt to determine if serotonin and Ca are working in a feedback loop to maintain blood Ca concentrations, we measured circulating serotonin in response to chelation of circulating Ca, in non-pregnant, nonlactating cows. We determined circulating serotonin concentrations differentially responded to the challenge, based on dietary Ca fed. Our data to this point indicates that serotonin and Ca are working in a feedback loop to regulate Ca homeostasis in the peripartal cow. We are currently working on dissecting pathways involved in the serotonin-Ca feedback loop and the contribution of the mammary gland to the regulation of this process.