Mammary epithelial cells synthesize and secrete norepinephrine (NE) into breast milk to regulate beta casein expression through the adrenergic beta(2) receptor. We investigated the expression, localization, and roles of NE transporter (NET) in the mammary epithelium during lactation. mRNA and protein levels of NET were determined in primary normal human mammary epithelial cells (pHMECs) and non-malignant human mammary epithelial MCF-12A cells. In nursing CD1 mice, NET localized to the apical membranes of the mammary epithelium. The intracellular NE content of pHMECs incubated with NE increased. Although the beta-casein concentration in milk was slightly higher at day 10 than at day 2 of lactation, the NE concentration and lactation-related proteins were only slightly changed on days 2-10. Restraint stress increased the NE concentration in milk from nursing mice and NET protein levels were significantly higher than in non-stressed nursing mice. NET is expressed on the apical membrane of mammary epithelial cells and incorporates NE in milk into cells, potentially regulating the NE concentration in milk. (C) 2021 Elsevier Inc. All rights reserved.