Since prolonged postoperative hypotony has been a frequent complication of glaucomatous filtration surgery in which mitomycin C (MMC) was used, it is important to determine the intraocular toxic effects of the drug. We placed MMC, in concentrations of 0.05, 0.2, and 1.0 mg/mL on the intact sclera of rabbits and obtained the eyes after 1, 7, and 28 days. Examination of the ciliary epithelium by transmission electron microscopy revealed an electron-dense material near the endoplasmic reticulum. Large intracellular vacuoles were present, and the mitochondria appeared swollen. These findings suggest a toxic effect of MMC on the ciliary epithelium in rabbits after extraocular application, possibly decreasing aqueous production.