Purpose To compare the biomechanical and histological properties of Achilles tendons repaired at different time points during the acute injury period. Methods Thirty-six skeletally mature Sprague-Dawley rats underwent bilateral mid-substance Achilles tenotomy. The Achilles tendons were repaired either in the first 24 h (group 1), 24-48 h (group 2), 48-72 h (group 3), or > 72 h (mean: 120 +/- 5.2 h) (group 4) after tenotomy. Six weeks after repair, nine tendons per group were assessed biomechanically and histologically. The Stoll histological scoring system was used for histological examination. The groups were compared with each other and native tendons (control group). The correlations between biomechanical and histological results were analysed. Results There were no significant differences between groups 1, 2 and 3 regarding the mean load to failure; it was significantly lower in group 4. Healed tendons in groups 1, 2 and 3 had significantly greater stiffness than native tendons and group 4 tendons. All healed tendons had a larger cross-sectional area than native tendons. There was no significant difference in tendon length between the groups. There was no significant difference in Young's modulus between the groups; Young's modulus was lower in all the groups than in the control group. Group 1 had significantly higher extracellular matrix organization, cell alignment, cell distribution and nucleus morphology scores and total scores than group 4. Group 1 had significantly higher extracellular matrix organization, cell distribution, vascularization and inflammation scores and total scores than group 3. A significant positive correlation was detected between the maximum load to failure and total histological score. Conclusion Repair of acute Achilles tendon rupture within 48 h, and especially in the first 24 h, provides better biomechanical and histological outcomes. In the clinical practice, the data could be used to decrease re-rupture rates, to achieve more anatomical tendon healing and to implement more effective post-operative rehabilitation programme.