6-Aminonicotinamide, given by intraperitoneal injection to male and female Dutch belted rabbits, produced swelling and vacuolation of ciliary and iridal epithelium plus vacuolation of the retinal pigment epithelial and outer plexiform layers of the retina. By transmission electron microscopy, inner and outer ciliary epithelial cells and inner iridal epithelial cells contained numerous coalescing, membrane-bound vacuoles of the cytocavitary network. These vacuoles were viewed as numerous interconnecting, intracytoplasmic cavities in scanning electron micrographs. Swelling of vacuolated epithelial cells and the presence of fibrin and proteinaceous fluid in the ciliary stroma resulted in thickening of the anterior ciliary processes with the formation of surface alterations detectable by scanning electron microscopy. In transmission electron micrographs the vacuoles in the retinal pigment epithelium were large, electron-lucent spaces and the vacuoles in the outer plexiform layer of the retina appeared to be intracytoplasmic spaces in axons of photoreceptor cells. Distention of cytocavitary structures has been reported in glial cells of animals given 6-aminonicotinamide and this change was apparently due to alterations in ion and water movement across cellular membranes that resulted in intracellular edema.