In experimental retinal detachment of the owl monkey we observed the formation of preretinal and subretinal membranes which had cellular connections to the retina proper. Based on light microscopic findings we concluded that retinal cells grow out of the retina and build up the preretinal and subretinal tissue. Electron microscopically these cells are identified as astrocytes which grow out through focal interruptions in the internal or external limiting membrane. In the immediate area of outgrowth Muller cell processes praticipate in the formation of the preretinal or subretinal tissue. These glial membranes can cause wrinkling of the internal limiting membrane and the nerve fiber layer or full-thickness folding of the retina. Glial proliferation plays an important role in the development of a complication in retinal detachment which we suggest calling "massive periretinal proliferation" (MPP).