A two-year-old female spayed Scottish terrier was presented to the Cornell University Hospital for Animals for acute onset of weakness, exercise intolerance, coughing, regurgitation, and vomiting. The patient was evaluated by a referring veterinarian, and was prescribed a nonsteroidal anti-inflammatory medication for possible right hind limb lameness. Worsening regurgitation and coughing during feedings prompted presentation to an Emergency hospital where complete blood count, chemistry panel, 4DX SNAP test, and thoracic radiographs were performed. The patient was diagnosed with Lyme disease, megaesophagus, and possible aspiration pneumonia. Failure to respond to antibiotic therapy and supportive care prompted referral to the Cornell University Neurology service the following day. On neurologic exam, the patient was mentally dull, markedly weak, and ambulatory paraparetic. Additionally marked, diffuse muscle atrophy, a decreased menace response OU, and an absent palpebral reflex OU were noted on exam. The patient was neurolocalized to the peripheral nervous system, and a neuromuscular junctionopathy was suspected. Additional diagnostics including thoracic radiographs, Tensilon response test, and acetylcholine receptor antibody titers were performed, which confirmed a diagnosis of Myasthenia gravis with secondary megaesophagus and aspiration pneumonia. Treatment with an acetylcholinesterase inhibitor and broad-spectrum antibiotics was initiated. Initial response to acetylcholinesterase inhibitor therapy was encouraging, and cyclosporine was added to the treatment regimen following resolution of aspiration pneumonia. At reevaluation six weeks following diagnosis, the patient was diagnosed with aspiration pneumonia, and was hospitalized for intravenous fluid and antibiotic therapy. The patient remained unresponsive to treatment, and persistent megaesophagus and worsening aspiration pneumonia ensued, necessitating repeated hospitalization for intensive management of aspiration pneumonia. Rescue therapy with mycophenolate mofetil was attempted, with no response. Given the lack of response to medical management and the poor long-term prognosis associated with this condition, euthanasia was elected. This report will describe pertinent clinical findings, diagnostics, and treatment in a dog with a peripheral nervous system lesion.