The effect of timing of analgesic drug administration on the severity of post-operative pain was investigated in dogs undergoing ovariohysterectomy using both subjective visual assessment scoring systems (VAS) and objective mechanical nociceptive threshold measurements using a novel handheld anti-nociceptiometric device. Forty dogs undergoing routine elective ovariohysterectomy were included in a randomised and double-blind study and assigned to one of three groups: (i) pre-operative analgesics; (ii) post-operative analgesics; (iii) no analgesics (saline injections). The analgesic used was pethidine (a short acting predominantly mu-opioid agonist), at a dose of 5.0 mg/kg (intramuscular). The post-operative administration of pethidine resulted in significantly higher sedation scores and significantly lower pain scores in the early post-operative period, but the dogs given pethidine pre-operatively had significantly lower pain scores than both the other groups at 8, 12 and 20 h post-extubation (P < 0.01, ANOVA). Mechanical thresholds measured at the distal tibia demonstrated the development of allodynia at 12 and 20 h post-extubation, and this was significantly prevented by the pre- (P < 0.01 at 12 h, P < 0.05 at 20 h, Kruskal-Wallis and post hoc Dunn's), but not by the post-operative administration of pethidine. Mechanical nociceptive thresholds measured at the ventral midline (site of surgery) demonstrated post-operative hyperalgesia in all groups; this hyperalgesia was least in the pre-operative pethidine group. In summary, this study clearly shows pethidine to be an effective analgesic in dogs, albeit of short duration of action, when administered post-operatively, and, importantly, that it has a positive benefit in terms of post-operative outcome measures, when administered pre-operatively, possibly as a result of blocking or preventing the development of central sensitisation following surgical stimulation.