The effect of loading on the early cellular response to tendon injury was studied in a partial tenotomy (window) model in the extensor digitorum longus of the rat. Normal use of the limb was allowed, such that tendons were either loaded (group 1) or unloaded (group 2) when a distal tenotomy was performed. Thirty-four male Fischer rats were used. The tendons were harvested at intervals of 6 hours and 1, 3, 5, and 7 days. Quantitative cell counts were performed on light microscopic cross-sections of the window and tendon substance together with recording of cell orientation. Matrix changes in the tendon, window, and tendon-window junction were observed on transmission electron microscopy. There was a rapid and extensive change in the tendon structure with rapid loss of definition of the window edge, and an increase in cellularity of the tendon substance. The loaded tendons demonstrated less cellularity at 5 days (window cell density 3.48 +/- 0.49 cells per 0.01 mm2) with better longitudinal orientation of cells and matrix than the unloaded tendons(8.38 +/- 1.1 cells per 0.01 mm2). The numbers of inflammatory cells in both groups were roughly comparable (5 days: loaded 0.411 +/- 0.071 cells per 0.01 mm2; unloaded 0.554 +/- 0.11 cells per 0.01 mm2), but the unloaded windows had more fibroblasts at 5-day and 7-day stages (5 days: loaded 3.08 +/- 0.44 cells per 0.01 mm2; unloaded 7.82 +/- 1.0 cells per 0.01 mm2; p < 0.016). Cell counts in the tendon substance were also higher in the unloaded (3.99 +/- 0.44 cells per 0.01 mm2) than the loaded (1.95 +/- 0.45 cells per 0.01 mm2) tendons at 5 days. This study demonstrated that the cellular response after injury in this extensor tendon model is affected by tensile loading, there being increased cell numbers in both the window and tendon substance in the unloaded tendon.