A six-year-old, spayed female, cocker spaniel was presented for hypoglycemic seizures. Hypoglycemia with concomitant hyperinsulinemia suggested an insulin-secreting tumor. Pancreatic masses were resected, and insulinoma was diagnosed. Six weeks later, the dog presented in hyperinsulinemic-hypoglycemic crisis (HHC). The dog was initially stabilized with intravenous dextrose boluses and infusions, but seizure activity recurred and persisted. A glucagon constant-rate infusion (GCRI) was initiated, and neurological signs quickly resolved. Dextrose was withdrawn over 24 hours, and euglycemia was maintained by GCRI alone. Despite aggressive medical management including the use of prednisone, diazoxide, bovine somatotropin, and streptozocin, the dog presented on two subsequent occasions in HHC and both times was rapidly stabilized with GCRI alone. In this dog, GCRI was a fast, safe, and effective method of achieving and maintaining euglycemia despite intractable hyperinsulinism. The clinical use of GCRI merits further investigation for management of HHC in veterinary species.