Lymphocytes are antinociceptive and can modulate visceral pain perception in mice. Previously, we have shown 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 β-endorphin release by T cells and an upregulation of β-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 β-endorphin expression; this was accompanied by a downregulation of the μ-opioid receptor (MOR). In vitro, T helper (Th) type 1 and type 2 cytokine stimulation of CD4+ T cells or isolation of T cells from in vivo Th-polarized mice did not increase T cell release of β-endorphin or the induction of β-endorphin expression in the myenteric plexus. However, exogenous β-endorphin did upregulate β-endorphin expression, and both cycloheximide and naloxone methiodide inhibited peptide upregulation. Therefore, our results suggest that nonpolarized CD4+ T cells release β-endorphin, which, through an interaction with MOR, stimulates an upregulation of β-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.