AtT-20 cells, which make and release beta-endorphin, or AtT-20/hENK cells, an AtT-20 cell line transfected with the human proenkephalin gene and secreting enkephalin as well as presumably beta-endorphin, were implanted in mouse spinal subarachnoid space. Cell implants did not affect the basal response to thermal nociceptive stimuli. Administration of isoproterenol, believed to stimulate secretion from these cells, produced antinociception in groups receiving AtT-20 or AtT- 20/hENK cell implants but not in control groups receiving no cells. The antinociceptive effect of isoproterenol was dose related and could be blocked by the opioid antagonist naloxone. Implantation of these cells offers a novel approach for the study of tolerance. Mice receiving AtT- 20 cell implants developed tolerance to beta-endorphin and the mu- opioid agonist DAMGO, whereas mice receiving genetically modified AtT- 20/hENK cell implants developed tolerance to the delta-opioid agonist DPDPE. Genetically modified AtT-20/hENK cell implants, but not AtT-20 cell implants, reduced the development of acute morphine tolerance in the host mice. This finding is consistent with the suggestion that enkephalin alters development of opioid tolerance. These results suggest that opioid-releasing cells implanted around mouse spinal cord can produce antinociception and may provide an alternative therapy for chronic intractable pain.