The morphology and permeability to horseradish peroxidase (HRP) of the ciliary muscle capillaries of squirrel monkeys were studied. The endothelial cells of the capillaries were of the continuous type and the interendothelial clefts were closed by a zonula occludens. Many vesicles were present in the cytoplasm of the endothelial cells. In addition to these vesicles, large vacuole-like structures, 100-500 nm in diameter, were observed in the cytoplasm of capillary endothelium, especially in the circular and radial ciliary muscles. When HRP was perfused into the anterior chamber, the 3,3'-diaminobenzidine reaction product was observed in the lumen of the ciliary muscle vessels after 30 minutes. When HRP was administered intravenously, the reaction product was observed among the muscle fiber bundles after 15 minutes. In both cases, the reaction products were also present in the cytoplasmic vesicles and vacuole-like structures, suggesting that these structures are involved in the bidirectional transport of HRP between interstitium and blood.