A 6-year-old, spayed female Labrador Retriever was referred to the Ontario Veterinary College, Veterinary Teaching Hospital (OVC-VTH) because of generalized weakness and ataxia. Progressive lethargy, exercise intolerance, and decreased interaction with family members were reported over the preceding 3 weeks. Decreased appetite, hypodipsia, vomiting, and left hind limb lameness subsequently developed. Abnormalities in serum electrolyte concentrations 1 week prior to examination included marked hypernatremia (193 mEq/L; reference interval [RI] 144-160 mEq/L) and hyperchloridemia (147 mEq/L; RI 109-122 mEq/L). Serum total thyroxine, thyroid-stimulating hormone, free thyroxine, triiodothyronine autoantibody, and thyroxine autoantibody concentrations and results of an ACTH stimulation test were within reference intervals.On examination, the dog was laterally recumbent with dull mentation and decreased responsiveness to external stimuli. In response to strong external stimuli, the dog could be roused to briefly ambulate. Mucous membranes were injected and tacky; 5-7% dehydration was estimated, and severe hypotension (60/43 mm Hg) a was detected. Submandibular and popliteal lymph nodes were mildly enlarged. Neurologic examination revealed abnormal mental status with dullness and disorientation. An absent menace response was detected in the left eye, with normal pupillary light reflexes and eyelid closure. Generalized weakness and proprioceptive ataxia were observed in all limbs, and were more pronounced on the left side. The neurologic lesion was localized to the right thalamocortex (diencephalon and telencephalon).Profound hypernatremia (193 mEq/L; RI 140-154 mEq/L) and hyperchloridemia (161 mEq/L; RI 104-119 mEq/L) with normokalemia (4.4 mEq/L; RI 3.8-5.4 mEq/L) were identified by serum electrolyte analysis. The serum was grossly lipemic, and biochemical abnormalities included mild hyperphosphatemia (6.1 mg/ dL; RI 2.8-5.7 mg/dL), hypermagnesemia (2.9 mg/dL; RI 1.7-2.4 mg/dL), hyperproteinemia (8.9 g/dL; RI 5.5-7.4 g/dL), hyperglobulinemia (5.2 g/dL; RI 2.1-4.2 g/ dL), and mildly increased urea concentration (71 mg/dL; RI 21-54 mg/dL) and alkaline phosphatase activity (181U/L; RI 22-143 U/L) with marked hypercholesterolemia (820 mg/dL; RI 139-394 mg/dL) and hypertri-