The antidiabetic drug metformin has demonstrated antinociceptive efficacy in different pain models, and these effects are usually attributed to activation of the AMP-dependent protein kinase (AMPK). However, the downstream targets that contribute to inhibition of nociception following AMPK activation have been only partially elucidated. Here, we examined the contribution of serotonergic mechanisms in mediating metformin's antinociceptive effects, seeing as AMPK activators (including metformin) have been shown to modulate serotonergic neurotransmission. The formalin test in mice was used as an inflammatory pain model. First, we examined metformin's effects following systemic (intraperitoneal) and local peripheral (intraplantar) administration. In the second part, we examined the roles of the AMPK and serotonin system in mediating metformin's antinociceptive effects by (locally and/or systemically) pretreating animals with the AMPK inhibitor (dorsomorphin), antagonists of serotonin 5-HT