An 8 year old F/S American Staffordshire Terrier presented with a previous history of PU/PD and recent onset of mental dullness, ataxia, and head pressing. Her laboratory evaluation revealed an elevated alkaline phosphatase, hyperchloridemia, hyponatremia and mild hyperglycemia. MRI of the brain was performed both with and without contrast. A large homogeneous mass was identified involving the pituitary gland, and extending dorsally to the ventral hypothalamus. The mass appeared hyperintense (bright) on T2 weighted images and enhanced minimally and heterogeneously, mostly prominently along the dorsal margin. Peri-mass edema, extending to the level of the thalamus was identified. Mass dimensions were approximately 16mm (l) x 16mm (w) x 10mm (h). The diagnosis is pituitary macroadenoma.