Simple SummaryPigs used in research are often subjected to procedures for which pain relief is crucial. However, the literature indicates that analgesics are under-reported and under-used in experiments involving pigs. Fentanyl is one of the opioids used to provide post-operative pain relief in experimental pigs. Analgesic requirements are often assessed using behavioral indicators such as activity. However, the effects of fentanyl on behavior in pigs are largely unknown. The aims of the current study were to investigate how fentanyl influences behavior in pigs, and if serotonin could be involved in fentanyl-induced behavioral side effects. The results of this study demonstrate that fentanyl can influence activity level, and induce different repetitive behaviors in pigs. Some of the behavioral changes induced by fentanyl seem to involve the serotonergic system. In conclusion fentanyl might influence behavior through mechanisms unrelated to analgesia. This is important for experimental pigs as it could interfere with pain assessment and determination of analgesic requirements.AbstractAnalgesic effects of fentanyl have been investigated using behavior. The behavioral effects of fentanyl and possible serotonergic influence are largely unknown. We therefore investigated behavioral effects of fentanyl, with or without the serotonin antagonist ketanserin, in pigs. Fourteen mixed-breed pigs, weighing 17–25 kg were included in a randomised blinded prospective, balanced three-group study. Ten pigs received first 5 and then 10 µg/kg of fentanyl intravenously. Ketanserin at 1 mg/kg or saline was given intravenously as a third injection. Four control pigs received three injections of saline. Behavior was video-recorded. The distance moved was automatically measured by commercially available software, and behaviors manually scored in retrospect. Fentanyl inhibited resting and playing, and induced different repetitive behaviors. The mean (SD) distance moved in the control group and fentanyl group was 21.3 (13.0) and 57.8 (20.8) metres respectively (p< 0.05 for pairwise comparison). A stiff gait pattern was seen after fentanyl injection for median (range) 4.2 (2.8–5.1) minutes per 10 min, which was reduced to 0 (0–4) s after ketanserin administration. Conclusion: fentanyl-induced motor and behavioral effects, and serotonergic transmission may be involved in some of them. The psychomotor side effects of fentanyl could potentially interfere with post-operative pain evaluation in pigs.