To determine the cause of intersegmental fusion--a fusion between the plasma membranes of the inner and outer segments--in vertebrate rod cells. Rod cells from adult salamander, frog, and rat retinas were examined with electron microscopy. The fine structure of fusion and the frequency of occurrence were assessed in three types of preparations (intact eyeballs, eyecup preparations, and neural retinas detached from the retinal pigmented epithelium). The effects of temperature, depolarization, and calcium were examined in isolated neural retinas by maintaining retinas in culture under defined conditions. Retinas from anesthetized animals also were examined. Intersegmental fusion in rod cells was consistently observed in all species after retinal detachment and maintenance in a standard Ringer's solution for 20 to 30 minutes. The amount of fusion seen in isolated amphibian retinas was sensitive to a number of factors: Elevated temperature increased the number of fused rod cells, whereas high KCl, low CaCl2, and Tricaine, which has calcium-related effects, reduced or prevented fusion. In attached retinas, fusion was seen infrequently; the incidence of fused cells in amphibian eyecup preparations, however, could be increased by incubation in the cold. The extensive continuity between the inner and outer segments created by their fusion did not disrupt intracellular structure. Moreover, the fusion seemed to be reversible with time. Intersegmental fusion is a feature of vertebrate rod cells. Although seen predominantly after retinal detachment, it is influenced by factors that affect other well-known exoplasmic membrane fusions. Thus, it may be one of the many regulated membrane fusion-fission events within the rod photoreceptor.