SCOPE: Peppermint is traditionally used as an antigalactagogue in breastfeeding women. However, the suppressive mechanism remains unclear. The authors investigate whether and how peppermint influences milk production at the cellular and molecular levels. METHODS AND RESULTS: A lactating mammary epithelial cell (MEC) culture model that produces major milk components is prepared. Peppermint oil (PMO) suppresses β-casein production in conjunction with the induced enlargement of cytoplasmic lipid droplets (CLDs). PMO also significantly inactivates STAT5 and mTOR in the lactogenic signaling pathway. Menthol treatment, which is a primary PMO component, leads to decreases in β-casein production, enlarged CLDs, the inactivated STAT5 and mTOR. Eucalyptol and menthyl acetate, other components of peppermint, does not show suppressive effects on lactating MECs. The inactivation of STAT5 and mTOR upon menthol administration is also evident in alveolar MECs of lactating mice. Furthermore, lactating MECs expresses TRPM8 and TRPA1, which are menthol receptors known as cold receptors. Agonists of TRPM8 and TRPA1 suppresses β-casein production and inactivation of STAT5 and mTOR in the lactating MECs. CONCLUSION: These findings indicate that peppermint has potential as an antigalactagogue. Menthol is suggested to be an active compound in peppermint. TRPM8 and TRPA1 may function as receptors for menthol.