Exposure to either cold or warm stress increased the thermal nociceptive thresholds of the terrestrial snail, Cepaea nemoralis. The warm stress-induced 'analgesia' was blocked by the prototypic opiate antagonist, naloxone, and the delta-opiate antagonist, ICI 154,129, and was suppressed by a 24-h pretreatment with the irreversible opiate antagonist, beta-funaltrexamine (B-FNA). In contrast, cold stress-induced analgesia was unaffected by either naloxone, ICI 154,129 or B-FNA. These results indicate that this mollusc displays both opioid and non-opioid forms of stress-induced analgesia in a manner analogous to that reported for mammals. These findings suggest an early evolutionary development and phylogenetic continuity of opioid and non-opioid mediated stress responses to aversive environmental stimuli.