['The aim of this study was to investigate the effect of a topical FK506 nanospheric suspension in a rat model of penetrating keratoplasty.', 'FK506 nanospheres were prepared by using a biodegradable poly (lactic-co-glycolic acid) copolymer (PLGA). Its distribution in the eye and blood after a single instillation was examined in rabbits. Sprague-Dawley (SD) rats received corneal heterografts and were topically treated with phosphate-buffered saline (PBS), PLGA, FK-506 0.01% (nanospheres), or dexamethasone 0.05% solutions twice a day for 28 days. Rejection index and graft-survival time were recorded and compared between the four groups. Three grafts were collected at different time points for immunohistochemical studies.', 'In the cornea, the FK-506 concentration reached its peak within 1 h of a single eye-drop instillation and then decreased by half (1667.85 +/- 611.87 ng/g) at 8 h. FK-506 cannot be detected in rabbit blood. There were significant differences in the graft-survival time between the FK-506 nanosphere group (15.09 +/- 4.81 days) and the other three groups [PBS (7.90 +/- 1.20, t = -4.594, P < 0.001), PLGA (8.44 +/- 0.88, t = - 4.074, P = 0.001) and dexamethasone (10.44 +/- 1.42, t = -2.790, P = 0.012)]. The rejected corneas in the FK506 nanosphere group showed significantly fewer CD4, CD8, CD68, CD79, vascular endothelial growth factor, ICAM, and tumor growth factor-beta(1)-positive cells than those in the other groups.'] Does preliminary study of the effect of FK506 nanospheric-suspension eye drop on rejection of penetrating keratoplasty?