Bovine tooth germs dissected from the mandible immediately after bleeding of the less than 1-yr-old calves (Bos taureus) were used to study the initial phases of giant tubule (GT) formation. Three stages could be discerned. Firstly, the initial manifestations were represented by loci about 150 micron apart in the axiomesiodistal plane along the pulpal border of the 100-200-micron-thick incisal dentin. Some of these loci showed only a blunt capillary loop and associated cells, whereas in most others there was a 50-100-micron-wide dense matrix consisting of coarse collagen fibrils and showing a metachromatic staining reaction to toluidine blue. Fibroblasts lined this matrix, and a capillary loop was always present underneath. Both kinds of GT origins were eventually circumvented by the odontoblasts situated incisal to them. The matrices became enclosed in the dentin. In the second stage, the GT grew in length as new dentin was formed. In this stage the vascular loops and the adjoining fibroblasts remained stationary within the GT lumina relative to the origins. In the third stage the GT vessels started to migrate pulpally relative to the origins. This seemed to be the result of a sequestration of the most incisal portions of the vascular loops. In this last stage the formation of collagen fibrils in the GT lumen incisal to the vessel loops started.