Two cases of a pheochromocytoma occluding the posterior vena cava are presented. One, in a 14-year-old wire-haired terrier, caused no clinical signs and was an incidental finding at necropsy. In addition to partial occlusion of the vena cava, this neoplasm had metastasized to the spleen. The other, in a 10-year-old male cocker spaniel, had a clinical syndrome resembling congestive heart failure and was characterized by ascites, edema of the posterior extremities, and weakness. Histologic evidence of hypertension was conclusive on both cases.