We have tried to answer the question of whether the endothelium of Schlemm's canal is derived from and retains properties of blood vessels by studying: (1) the development of Schlemm's canal in human fetal eyes; (2) the existence of Weibel-Palade bodies in human neonatal, adult human and adult monkey eyes; and (3) the presence of blood coagulation Factor VIII-related antigen in adult human and monkey eyes. (1) We observed that the intrascleral plexus of the limbal region extended deep into the sclera forming a deep scleral plexus by the 17th week of gestation. After 17 weeks gestation, extensions from the deep scleral plexus had reached the region of the future corneoscleral meshwork where the trabecular cells appeared oriented circumferentially. The blind endings of these extensions appeared to grow circumferentially in the supposed region of Schlemm's canal and at 27 weeks gestation they formed an incomplete Schlemm's canal. A complete Schlemm's canal was observed in some sections of the limbal region at 28 weeks gestation and at approximately 40 weeks gestation the canal was complete in most sections. (2) Weibel-Palade bodies were found in the endothelium of aqueous veins and in the inner and outer wall endothelium of Schlemm's canal. (3) Blood coagulation Factor VIII-related antigen was detected in the endothelium of the collector channels and Schlemm's canal, as well as in the blood vessels of the other parts of the eye. Our results indicate that the endothelium of Schlemm's canal is derived from a vascular origin and that even in the adult eye it retains some of the properties of a blood vessel.