Maternal physiology of dairy cows shifts at the onset of lactation to adapt to the immense nutrient demands by the mammary gland, resulting in altered tissue metabolism. Research over the last decade has focused on understanding the shift in calcium metabolism, with an emphasis on the relationship of serotonin with calcium during the transition period and lactation. In a new review in the December issue of the Journal of Dairy Science, researchers examine the current knowledge on calcium metabolism, mammary calcium transport, serotonin metabolism, and the serotonin-calcium axis.