The process of the arrangement of D-periodic collagen fibrils and their growth in maturing tail tendon of mice were studied with the association of type VI collagen, from fetal day 10 to 10 weeks after birth. In tail tendons, the amount of collagen fibers gradually increased along with the diameters of D-periodic collagen fibrils during maturation. Type VI collagens first appeared on fetal day 10, when D-periodic collagen fibrils were not recognizable. Type VI collagens were observed around the fibroblastic cells in early stages of development, but were among thick collagen fibrils in the adult tendon. While the periodic distances of type VI collagen fibrils were over 100 nm at fetal days, they were packed to 80-90 nm after birth. The periodic bands were stained well with ruthenium red in adult but not in young tendons, indicating the close association of proteoglycans or glycosaminoglycans (PGs/GAGs) with maturing type VI collagens. Since type VI collagen in native form is known to associate with D-periodic collagen fibrils via PGs/ GAGs, ruthenium red-stainability on the surface of D-periodic collagen fibrils was also examined; results showed that ruthenium red-stainable elements were D-periodically associated. When the surface morphology of D-periodic collagen fibrils in adult animals was examined by atomic force microscopy, a large depth of the groove between elevated and depressed surfaces became prominent when the fibril surface was digested with hyaluronidase. Thus, it is possible to observe topologically the association of PGs/GAGs and probably that of type VI collagens with D-periodic collagen fibrils.