Compared to rats that were given three daily sessions of habituation to a laboratory environment, non-habituated rats were behaviourally less reactive when pain was produced by 0.05 ml of 2.5% formalin injected s.c. in one hindpaw. This behavioural analgesia was equivalent to that produced by approximately 2.0 mg/kg morphine and was interpreted as analgesia induced by the stress of exposure to a novel environment. Methysergide (10 mg/kg) or 1-valine (200 mg/kg) alone, or combined with naloxone (0.1 or 2.0 mg/kg), reversed the effect of exposure to a novel environment. Naloxone itself had no significant effect. In habituated rats 1-valine reduced the response to formalin-induced pain whereas methysergide combined with naloxone increased it. It is concluded that analgesia induced by the stress of exposure to a novel environment depends on serotonin.