<p>Psychological stress during lactation decreases β-casein concentration in milk. Smoking-induced increases in miR-210 expression also decrease β-casein concentration in milk and inhibits its synthesis in mammary epithelial cells. However, the relationship between stress and miR-210 expression in the mammary epithelium has remained uninvestigated. Here, we investigated the association between stress and miR-210-5p (miR-210) expression in the mammary epithelium and evaluated whether miR-210 is involved in stress-induced suppression of β-casein content in breast milk. miR-210 expression in the milk and mammary epithelium of restraint-stressed mice was significantly higher than that in control mice. Additionally, β-casein mRNA expression and the protein levels of signal transducer and activator of transcription 5 (STAT5) and phosphorylated STAT5 were significantly lower than those in control mice. In clinical investigation, a significant positive correlation was observed between miR-210 levels in breast milk and the total mood disturbance (TMD) score determined using the Profile of Mood State 2nd Edition–Adult Short questionnaire. In contrast, the positive mood scale vigor–activity on the TMD subscale and the positive mood scale friendliness exhibited a significant negative correlation with miR-210 levels in breast milk. These results indicate that miR-210 levels in breast milk may be increased in nursing mothers with a strong negative mood. In conclusion, psychological stress during lactation may induce miR-210 expression in the mammary epithelium, resulting in increased miR-210 levels in breast milk. Additionally, stress-induced suppression of β-casein concentration in breast milk might be related to the induction of miR-210 expression in the mammary epithelium.</p>