Ciliary process vasculature has an important role in aqueous humor production. There have been, however, few reports describing the overall cytoarchitecture of ciliary process vasculature. The wall cytoarchitecture of microvessels in the rat ciliary process was elucidated by scanning electron microscopy after removal of ciliary epithelia and connective tissue components with HCl hydrolysis. Utilizing characteristics of cellular morphology and vessel diameters, several vascular components were identified along the vascular tree: 1) arterial iridociliary circles (30-60 microm in outer diameter), containing a compact layer of circularly oriented spindle-shaped smooth muscle cells; 2) the proximal part of the radial ciliary arteriole (10-25 microm), containing a less compact layer of circularly oriented branched-smooth muscle cells and spindle-shaped smooth muscle cells; 3) a middle part of the radial ciliary arteriole (20-35 pm), with circularly oriented branched-smooth muscle cells and irregularly oriented stellate cells with ramifying projections; 4) a distal part of the radial ciliary arteriole (10-20 microm), possessing irregularly oriented stellate cells with ramifying projections; 5) marginal venules (15-20 microm), with spidery pericytes possessing highly ramifying and overlapped projections; 6) capillaries in the ciliary process (4-7 microm), with widely scattered pericytes having longitudinal and several circular projections; 7) venules in the posterior basal region of the ciliary process (greater than 5 microm), with widely scattered pericytes having a few thin projections. From arterial iridociliary circles to venules in the basal region of ciliary process, seven parts could be recognized by wall cytoarchitecture, which was discussed in relation with the function.