The morphology of vessels of the cervical rete mirabile near the region of the foramen magnum of the bowhead whale, Balaena mysticetus, as well as of retial vessels of the mandibular foramen was studied by light and transmission electron microscopy. A comparison of arterial and venous components of the mandibular rete revealed considerable arterial branching and arteriovenous anastomoses. Although the small arteries of bowhead retia contained the same layers found in nonretial arteries, several distinctive morphologic features were evident. These included wide separation by collagenous connective tissue and small size of individual smooth muscle cells of the tunica media. These myocytes also contained considerable perinuclear glycogen and communicated minimally with adjacent myocytes by small foot-like cytoplasmic appendages as well as branching of basement membranes. Ganglia-like neural plexuses were observed within the tunica media of arterial retia. Endothelial cells lining the intima demonstrated loose peripheral edges which frequently projected into the vessel lumen, and these cells rested upon a subjacent layer of reticular fibers. It is known for some species that the retial supply to the brain in whales largely supplants an internal carotid arterial supply and that the spinal cord is supplied by retia. The physiologic role of the bowhead mandibular retia remains unknown. The retia mirabilia of cetaceans are considered as one of the morphologic adaptations which modulate hemodynamics during diving and resurfacing. The morphologic features of these vessels in the bowhead whale, as revealed by electron microscopy, appear to reflect the capability to respond in a slow but sustained manner to diving, and the large glycogen deposits may help sustain vascular myocyte metabolism during prolonged submersion.