Lymphocytes are anti-nociceptive and can modulate visceral pain perception in mice. Previously we showed that adoptive transfer of CD4⁺ T cells to severe combined immune deficient (SCID) mice normalized immunodeficiency-related visceral hyperalgesia. Pain attenuation was associated with an increase in -endorphin release by T cells and up-regulation of -endorphin in the enteric nervous system. In this study, we investigated the relationship between T cells and opioid expression in the myenteric plexus. We examined opioid peptide and receptor expression in the myenteric plexus in the presence and absence of mucosal T cells. We found a positive association between T cells and -endorphin expression; this was accompanied by a down-regulation of mu opioid receptor (MOR). In vitro, Th1 and Th2 cytokine stimulation of CD4⁺ T cells or isolation of T cells from in vivo Th-polarized mice did not increase T cell release of -endorphin or induction of -endorphin expression in the myenteric plexus. However, exogenous -endorphin did up-regulate -endorphin expression and both cycloheximide and naloxone methiodide inhibited peptide up-regulation. Therefore, our results suggest that non-polarized CD4⁺ T cells release -endorphin, which through an interaction with MOR, stimulates up-regulation of -endorphin expression in the myenteric plexus. Thus we propose that the mechanism underlying lymphocyte modulation of visceral pain involves T cell modulation of opioid expression in the enteric nervous system.