The effects of paracentesis on the blood-aqueous barrier of cynomolgus monekeys were studied by light and electron microscopy. The following regions of the eye were examined: ciliary body, iris, Schlemm's canal, juxtacanalicular connective tissue, and trabecular meshwork. Prominent structural alterations were seen in the ciliary epithelium at the anterior portion of the pars plicata, but the epithelium at the posterior portion of the pars plicata and at the pars plana appeared less disrupted. Plasma proteins which crossed the capillary endothelium accumulated in the stroma, and appeared in the posterior chamber through the enlarged extracellular spaces of the ciliary epithelium. The plasma proteins in the ciliary stroma of the anterior portion of the pars plicata moved to the stroma of the iris root; whereas in the iris the capillary endothelium provide an impermeable barrier to proteins. Following paracentesis, the lumen of Schlemm's canal was invaded by blood, and through newly formed gaps in the endothelial lining of the inner wall of the canal, the plasma proteins and red blood cells rapidly diffused into the juxtacanalicular connective tissue, trabecular meshwork, and anterior chamber. On the basis of the present experiments, the protein of the aqueous humor can be considered as originating from both the anterior portion of the pars plicata and the inner wall of Schlemm's canal.