The behavior and role of blood vessels and blood-borne cells in the process of the remodeling of the periodontal ligament (PDL) incident to experimental tooth movement was studied in rats. Particular interest was focused on areas of tension and of pressure with frontal bone resorption but without overt hyalinization. An increase of vascular activity occurred in the above mentioned situations. Extensive breakdown of collagen was observed in pressure areas with frontal resorption and in areas of tension concomitant with vascular invasion. Two patterns of fiber and bone remodeling were seen in areas of tension: intense vascular activity within the periodontal membrane and intense vascular activity inside the alveolar bone. Macrophages occurred consistently near blood vessels both in areas of tension and in areas of resorption. These are multipotent cells that obviously influence the remodeling process.