- Purpose: The effects of subconjunctival injection of mitomycin C and different concentrations of 5-fluorouracil on the epithelium of the ciliary body of twenty Sprague Dawley SD rats was studied.Methods: Twenty rats were divided into four treatment groups.The first three groups received 0.2 ml of 5, 10, and 30 mg of 5-fluorouracil subconjunctivally respectively, and the fourth group 0.2 ml of 0.4 mg/ml mitomycin C subconjunctivally. The right eye received 0.2 ml of the antimetabolite,while the left eye was injected with 0.2 ml of saline subconjunctivally, as a control.The eyes were examined histologically, in a masked fashion, by light and bytransmission electron microscopy. In each treatment group, two eyes were examined after one week, and three eyes were examined one month after the subconjunctival injection. Results: Electron microscopy revealed toxic effects in the epitheliumof the ciliary body of all treatment groups. The 5-fluorouracil group revealed focal mitochondrial edema, enlargement of intercellular spaces, and dilatation of intracellular spaces. The mitomycin C group showed pyknotic nuclei,enlargement of intercellular spaces, and irregular flattened epithelial cells. Theseverity of changes correlated with concentration and length of exposure. No pathology was found by light microscopy in all groups.Conclusions: This study demonstrates that subconjunctival antimetabolites mitomycin C and 5-fluorouracil can penetrate the sclera and exert toxiceffects on the epithelium of the ciliary body, even in low doses. These changeswere only apparent by electron microscopy and were still present one month after theinjection. These findings may contribute to the theory that the application ofantimetabolites during or after surgery has a direct effect on the epithelium of the ciliary body, besides its known effect on the conjunctiva. Further studies are needed to evaluate its effect on intraocular pressure.