The treatment of Achilles tendon rupture is controversial between surgical repair and conservative cast treatment. This study was attempted to compare the biomechanical and morphological results between operative and nonoperative treatments of experimentally tenotomized rabbit Achilles tendon with the lapse of time. A total of 72 adult rabbits were used. After tenotomizing the Achilles tendons, the subjects were divided into 3 groups according to the time of the initiation of the treatment and each group was subdivided into 2 further subgroups according to the method of treatment; immediate operation and nonoperation, 1 week delayed operation and nonoperation, and 2 weeks delayed operation and nonoperation subgroups. Ten animals from each subgroup were sacrificed after four weeks of cast immobilization and the Achilles tendons were prepared for the gross, biomechanical and histological examinations. At the same time, two animals from each subgroup were examined by Microfil perfusion for microvascular changes in the healed tendons. On biomechanical examination, maximal loads were decreased with the lapse of time in each treatment method, and there were statistical significances between the immediate and 2 weeks delayed operation subgroups, 1 week and 2 weeks delayed operation subgroups, and immediate and 2 weeks delayed nonoperation subgroups. A maximal loads were higher in the operative treatment of each group but there was no statistical significances between operative and nonoperative treatment of each group. The results of absorption energy and stiffness were similar to those of maximal load. In conclusion, these results suggest that a rupture of the Achilles tendon should be treated as early as possible preferably within 1 week, in order to achieve a high tensile strength irrespective of the treatment method. In terms of rerupture, nonoperative treatment is comparable with surgical treatment if a rupture of Achilles tendon is managed within 2 weeks of injury.