Injection of a hyperosmotic solution into the rabbit vitreous will cause rapid detachment of the retina. Osmotic detachment was demonstrated in the monkey eye. No anatomic changes accounted for the early separation of the retina, but within a few min of detachment in the rabbit or several h in the monkey [cynomologus or rhesus monkey], the retinal pigment epithelium [RPE] microvilli lose their normal organization. This may be a nonspecific response to separation from the outer segments. After longer periods of osmotic detachment in the rabbit, the RPE cells became edematous and the microvilli shrank and disappeared. Although some cells ruptured at the apical membrane and large cysts were often observed above the cell junctions, the intercellular tight junctions always appeared intact. Osmolarity requires consideration in the evaluation of intravitreal injections for man. Care should be used to avoid injection of concentrated solutes near the retina.