Native opioid peptides serve as growth factors in a number of normal and neoplastic cells and tissues. This study investigated the influence of opioids on circadian rhythm-dependent DNA synthesis in mouse esophagus during homeostatic renewal. In contrast to a labeling index (LI) of 24.0% at 0630 and 5.5% at 1600, disruption of opioid-receptor interaction by the potent opioid antagonist naltrexone hydrochloride (NTX; 10 mg/kg) in mice resulted in an elevation of 49% in DNA synthesis of esophageal epithelial cells at 1600, but had no effect at 0630. Mice subjected to [Met5]enkephalin (1 mg/kg) had an LI that was decreased 23% from control levels at 0630, but was unaffected at 1600. This decrease in DNA synthesis was blocked by concomitant administration of naloxone (10 mg/kg); naloxone alone had no influence on cell replicative processes. In tissue culture studies, NTX and OGF markedly increased and decreased, respectively, the LI from control values. Both opioid growth factor (OGF) and its receptor, zeta, were detected in all but the cornified layer of mouse esophageal epithelium and in the epithelial cells of the stomach and small and large intestines. In addition, both peptide and receptor were observed in the basal and suprabasal cells of human esophageal epithelium. These results indicate that an endogenous opioid peptide (OGF) and its receptor (zeta) reside in gastrointestinal epithelium and play a role in cellular renewal processes in a tonically inhibitory, direct, and circadian rhythm-dependent fashion.