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SUMMARY Purpose: To evaluate histopathological changes occuring in corneal endothelium after injection of trypan blue in different concentration into anterior chamber of rats. Methods: In this experimental study, 40 eyes of 20 Wistar albino rats were used. Trypan blue in different concentrations were injected into right eyes of the rats and 0.05 ml balanced salt solution (BSS) into left eyes (control group). Right eyes were grouped into 5, each consisting 4 eyes. Each group received 0.05 ml trypan blue in 0.4%, 0.2%, 0.1%, 0.05%, 0.025% concentrations respectively. One day and one month after injection two rat from each trypan blue concentration were sacrified and their eyes were enucleated. Corneal endothelial cells of enucleated eyes were analysed with transmission electron microscopy (TEM) and the results were compared with the control group. Results: Trypan blue, more evidently in 0.4% and 0.2% concentrations, vacuolisation in cells, intracytoplasmic edema, extensive granulated endoplasmic reticulum, nucleus picnotic, and crystalysis in mitokondrions. In the late period, these changes persisted in a decreasing manner. In 0.1% and 0.05% concentrations, nucleus was seen normal, granulated endoplasmic reticulum and golgi were seen actively, at minimal levels crystalysis in mitokondrions were observed. No differance in both early and late periods was determined between 0.025% concentration and control group. Conclusion: : Trypan blue markedly in 0.4% and 0.2% concentrations and in 0. 1% and 0.05% concentrations causes structural changes in corneal endothelial cells. In 0.025% concentration, ultrastructural building of endothelium was not different from control group. 40