• The treatment of rats with cyclosporine A by the oral, intraperitoneal, or intravenous routes results in a high level of the drug within the liver and spleen in all cases. The blood level varies and is highest after intravenous injection. The eyes of these animals do not show any significant accumulation of the drug, either extraocularly or intraocularly except for the rats treated by intravenous injection, where radioactive cyclosporine A is detected within the chorioretinal complex. Even in these cases, however, no radioactivity is observed within the vitreous. After local application of 2% cyclosporine A eye drops in olive oil, a high concentration of the drug is found in the cornea and conjunctiva only. No intraocular penetration is observed in these cases. However, when cyclosporine A eye drops in absolute alcohol are used, high levels of the drug are also found in intraocular tissues. Thus, in the intact eye, the cornea and conjunctiva on the one hand and the blood retinal barrier on the other hand prevent the intraocular penetration of cyclosporine A.