We recently showed that -casomorphin-7, a milk opioid peptide, strongly stimulates mucin secretion in rat jejunum through a nervous pathway and opioid receptor activation. In this study, the hypothesis that -casomorphin-7 may also act directly on intestinal goblet cells was investigated in vitro in rat and human intestinal mucin-producing cells (DHE and HT29-MTX) using quantitative and semi-quantitative RT-PCR and ELISA. The presence of µ-opioid receptors was demonstrated in rat goblet cells in the upper half of the colonic crypt and in the two cell lines by immunohistochemistry and RT-PCR. In rat DHE cells, -casomorphin-7 increased the expression of rMuc2 and rMuc3 but not of rMuc1, rMuc4 and rMuc5AC. This effect was time- and dose-dependent, the maximum of increase in transcripts being noticed for a concentration of 10^-4 mol/L after 2 hours of stimulation for rMuc2 (225 % of controls) and 4 hours of stimulation for rMuc3 (208 % of controls). Mucin secretion was maximally increased after 8 hours of stimulation. Interestingly, these effects were prevented by pre-treatment of the cells with the µ-opioid antagonist cyprodime. In human HT29-MTX cells, -casomorphin-7 (10^-4 mol/L) also increased MUC5AC mRNA levels (219% after 24 hours of stimulation) and the secretion of this mucin (169% of controls). In conclusion, -casomorphin-7 may contribute significantly to mucin production via a direct effect on intestinal goblet cells and the activation of µ-opioid receptors. Since intestinal mucins have a crucial mucosal protective function, dairy products containing -casomorphin-7 may improve intestinal protection and could have dietary and health applications.