The United States is battling a drug overdose crisis fueled by deaths from the synthetic opioid fentanyl and, more recently, the veterinary sedative xylazine, an alpha-2 adrenergic receptor agonist drug. A better understanding of the toxicity caused by these drugs, alone and in combination, will guide strategies to prevent and to medically manage victims of these drugs. Fentanyl causes death through respiratory depression, however little is known how or if xylazine and fentanyl interact to produce additional toxicity. By itself, xylazine in both humans and animals, causes sedation, respiratory depression, hypotension, bradycardia, and hypothermia. In Aim 1, therefore, we will quantify the impact of fentanyl on xylazine-induced hypotension, bradycardia, and respiratory depression in rats. Because fentanyl causes skeletal muscle rigidity and airway compromise/glottic closure, which may further impair breathing, and, because other alpha-2 adrenergic receptor agonist drugs (e.g., dexmedetomidine) inhibit opioid-induced rigidity, we will also measure xylazine skeletal muscle effects. Alpha-2 adrenergic receptor agonist drugs, like other sedatives and anesthetics, impair thermoregulation and decrease thermogenesis and metabolism, which promotes hypothermia. We hypothesize xylazine causes marked hypothermia through similar mechanisms and will test this in Aim 2. Xylazine is also associated with skin wounds in intravenous drug users. Because hypothermia is known to impair wound healing and to increase wound infection rates, we further hypothesize xylazine-induced hypothermia, perhaps exacerbated by fentanyl/opioid use, may be contributing. To test this last hypothesis, in Aim 2 we will determine xylazine, fentanyl, and systemic warming effects in an established rodent contaminated wound healing model.