Alterations in collagen fibril diameter distribution were measured in rat extensor tendons subsequent to two-successive partial tenotomies, 30 days apart. Subsequent to surgery three areas were created. The primary lesion area, consisted of a regenerating window in the central half of the tissue. Secondary lesion areas consisted of strips at the lateral margin of the tissue, removed during the second operation. Between the lesion areas were the non-lesion areas consisting of tissue not removed during surgery. Tissue was examined 40, 60, 120 and 240 days after the initial operation. Within each group, primary and secondary lesion areas remained unimodal in collagen fibril diameter distribution throughout the period of the study. This distribution was initially unimodal in the non-lesion areas but became multimodal by 240 days postoperatively. Both the primary lesion area and non-lesion areas exhibited an increase in frequency of small diameter fibrils between 40 and 60 days postoperatively. It is suggested that all observations may be related to the level of applied stress.