The collagen structure of young and old rats was examined using a scanning force microscope (SFM). Rat tail tendons of 8- and 24-month-old Wistar rats were frayed by two blades and examined using a Nanoscope III SFM. In the same tendons, the pentosidine concentrations, a marker of the Maillard reaction, were determined by HPLC. The SFM inspection of native fibrils produces images of collagen bundles, with parallel fibrils. The diameters of old rat collagen fibrils were large in comparison to the young ones. Moreover, fibrils obtained from old rats exhibited the same band interval, while the depth of the gap between two overlap zones showed a higher mean value with respect to young collagen. The pentosidine concentration was also higher in the old than in the young tendons. In conclusion, in the presence of an increased concentration of advanced glycation end products, significant structural alterations have been observed in old fibrils.