Six owl monkeys were exposed intravitreally for 6 to 28 months to Dow Corning Medical Fluid 350 in a morphologic study of silicone retinopathy. Fellow eyes of each silicone-injected animal and six eyes from three normal untreated monkeys served as controls. In addition, two eyes were treated post-mortem with silicone oil to insure that no artefact redistribution occurred. Each case was studied histochemically in frozen sections, with electron microscopy of Epon-embedded materials. All silicone-injected eyes showed numerous empty spaces in the inner segment of the retina adjacent to the vitreoretinal interface, corresponding to the intercellular spaces between Müller's filbers. These spaces contained numerous spherical bodies encircled by a homogeneous electron-opaque material, suggesting a silicone-phospholipid complex. Marked degeneration of small ganglion cells was seen. Müller's fibers appeared to be shrivelled. The lack of phospholipids in the inner segment of the visual receptors was marked, although no significant morphologic lesions were detectable. Abnormally high ATPase activity was found in both plexiform layers. There were no such lesions in the untreated control eyes, nor any detectable artefactual redistribution of silicone oil in the eyes treated post-mortem with Medical Fluid 350.