Purpose:To investigate the retinal toxicity of FK506 by intravitreal administration.Methods:Twenty-two eyes of 14 New Zealand rabbits were investigated. FK506 at concentrations of 5,25 and 50μg/eye was injected into the vitreous cavities respectively. The control eyes were received mixed solution of balanced salt and ethanol. All eyes were examined by tonometry, slit lamp and indirect ophthalmoscopy preoperatively and postoperatively at the 1st, 3rd, 7th,and 14th day respectively. In the final examination, all eyes were enucleated and processed for light and electron microscopy. Results:No evidence of toxic reaction was seen in the eyes received 25μg FK506 or less of FK506. Several eyes received 50μg FK506 and control eyes developed conjunctival congestion and slightly bloody exudates in anterior chamber which may be related to irritation of ethanol. Two of five eyes received 50μg developed transient vitreous opacities. Electron microscopically, the mitochondria of the photoreceptor cells were swelled in the eyes treated with 50μg FK506.Conclusion:It is safety with intravitreal FK506. There are no irritation and toxicity to the rabbits eyes with the intravitreal doses of 25μg FK506 or less. The doses of 50μg FK506 are proved to be toxic to the retina. Eye Science 2004;20:34-38. Purpose: To investigate the retinal toxicity of FK506 by intravitreal administration. Methods: Twenty-two eyes of 14 New Zealand rabbits were investigated. FK506 at concentrations of 5,25 and 50 μg / eye was injected into the vitreous cavities respectively. The control eyes were All eyes were examined by tonometry, slit lamp and indirect ophthalmoscopy preoperatively and postoperatively at the 1st, 3rd, 7th, and 14th respectively respectively. In the final examination, all eyes were enucleated and processed for light Results: No evidence of toxic reaction was seen in the eyes received 25 μg FK506 or less of FK506. Several eyes received 50 μg FK506 and control eyes developed conjunctival congestion and slightly bloody exudates in anterior chamber which may be related to irritation of ethanol Two of five eyes received 50 μg of transient transient vitreous opacities. Electron microscopically, the mitochondria of the photorecept or cells were swelled in the eyes treated with 50 μg FK506 or less. There are no irritation and toxicity to the rabbits eyes with the intravitreal dose of 50 μg FK506 or less. The doses of 50 μg FK506 are proved to be toxic to the retina. Eye Science 2004; 20: 34-38.