OBJECTIVE: To describe a case of a suspected anaphylactoid reaction to mannitol in a dog. CASE SUMMARY: A 1-year-old, neutered female French Bulldog was anesthetized for a T12-T13 dorsal laminectomy surgery for a previously diagnosed spinal arachnoid diverticulum. Perioperatively, the spinal cord appeared grossly edematous, and IV mannitol administration was elected. Midway through administration of mannitol, peracute erythema and angioedema were noted around the patient's tongue, muzzle, and paws. The dog's tongue grew to approximately twice its normal size. All vital parameters remained within normal limits. The dog was treated intravenously with dexamethasone and intramuscularly with mepyramine maleate. The remainder of the anesthetization was uneventful. The swelling completely resolved during the 2-hour recovery period, but careful management of the enlarged tongue was required to prevent airway occlusion. NEW OR UNIQUE INFORMATION PROVIDED: This is the first reported case of a suspected anaphylactoid reaction to IV administration of mannitol in a dog.