Giant tubules in the axiomesiodistally extended incisal dentin of unerupted permanent bovine incisors from 1/2-1 1/2-yr-old calves were studied by TEM. The pulpal portion of each giant tubule contained two capillaries forming a loop incisally. The capillaries consisted of a delicate, continuous, or fenestrated endothelium and often had a pericyte coating. Cells with a rich RER, mitochondria, and Golgi apparatus dominated along and just incisal to the vessels. Extravasated erythrocytes and occasional macrophages were also seen in the latter location. It was suggested that the pulpal retraction of the giant tubule vessels during primary dentinogenesis might be the result of an intermittent sequestration of the most incisal portion of the loop, concomitant with endothelial proliferation in the pulpal portion of the capillaries. The major portion of the giant tubules, incisal to the vessels, contained a dense matrix of more or less axially oriented unmineralized 0.07-0.1-micron-thick collagen fibrils and single cells showing gradually increasing degenerative and necrotic changes in an incisal direction. Longitudinal sections of the mineralized tubular walls revealed transversely cut collagen fibrils. The incisally located giant tubule origins were circumvented by dentinal tubules and contained 0.10-0.22-micron-thick, densely packed collagen fibrils with an electron dense perifibrillar zone.