The imidazoquinolines are immune response modifiers that have potent antiviral and antitumor properties. The mechanism by which they exert their effects on cell replication has been investigated in vitro and is related to the upregulation of the opioid growth factor receptor (OGFr) and modulation of opioid growth factor (OGF; [Met(5)]-enkephalin). The OGF-OGFr axis regulates cell proliferative events through a cyclin-dependent kinase inhibitory pathway. The present study examined the mechanism whereby imiquimod repressed cell proliferation in vivo. Using a nude mouse model that has a compromised T-cell immune system, as well as C57BL/6 mice with an intact immune system, the effects of topical imiquimod (Aldara(®)) on DNA synthesis of basal epithelial cells in skin were examined. Imiquimod's effects on DNA synthesis were detected 24 h after application, and could be observed for one week after a single treatment. The magnitude of change in DNA synthesis following imiquimod was similar for one, three or six applications. Naloxone, an opioid antagonist, blocked the inhibitory effect of imiquimod. Imiquimod in combination with OGF or a low dose of naltrexone (LDN; known to upregulate the OGF-OGFr axis) had no greater inhibitory response on DNA synthesis than either OGF or LDN alone. Both OGF and OGFr were upregulated in basal epithelium after imiquimod treatment. Both nude and C57BL/6 mice exhibited the same repressive action of imiquimod on epithelial DNA synthesis. Imiquimod was neither an opioid agonist nor antagonist using nociceptive testing, and did not induce apoptosis or necrosis. Exposure to imiquimod was found to depress DNA synthesis in cells located in distant epithelium from day 3 and lasted until day 5. These results suggest that the target of imiquimod on DNA synthesis is dependent on an opioid receptor-mediated pathway, and infers that imiquimod is reliant on the OGF-OGFr axis for modulating cell proliferation.