Induction of c-fos in the spinal cord due to pain is well established. This study aims to look at the effects of acute swim stress on Fos-like immunoreactivity (FLI) induced by formalin and how it is modulated by ketamine and morphine. Acutely-stressed and non-stressed adult male Sprague Dawley rats were pretreated with intraperitoneal injection of ketamine 5 mg/kg (Ketava, Atlantic Lab), morphine 10 mg/kg (Rhotard, Custom Pharmaceutical), or saline, 5 minutes prior to experimentation. Rats were acutely stressed by swimming for 3 min in 20 degrees C water. Dilute formalin (Formaldehyde, Merck) was injected to the hindpaw and the formalin score recorded. Rats were then sacrificed and spinal cords (L4-L5) removed for immunohistochemical analysis of FLI. Two-way ANOVA showed significant effects of stress, drug and stress-drug interactions in formalin test and FLI. Both morphine and ketamine produced analgesia in the formalin test. In the saline stressed group, FLI was suppressed on the ipsilateral side (p<0.01) but increased on the contralateral side (p<0.01) compared with non-stressed saline. In morphine and ketamine stressed groups, FLI was increased on both ipsilateral and contralateral sides for morphine (ipsilateral: p<0.05; contralateral: p<0.001) and ketamine (ipsilateral: p<0.05, contralateral: p<0.05) compared with their corresponding non-stressed groups. In conclusion, presence of stress may lead to discrepancy between behavioural manifestation of pain and c-fos induction in the spinal cord.