To determine the fate of rhodopsin during endocytosis-mediated retinal degeneration.Drosophila stocks were raised in complete darkness and shifted to light for 24 h prior to dissection and fixation of retinas. 1 microm frozen sections were cut on an ultracryomicrotome, then stained with antibodies specific for rhodopsin or arrestin. Localization of photoreceptor cell-specific proteins was determined by confocal microscopy.Flies that are in the process of undergoing endocytosis-mediated retinal degeneration exhibit an apparent loss of rhabdomeric rhodopsin at early times during the degenerative process. Using different immunological agents, genetic backgrounds, and light treatments, we have found that the binding of arrestin to rhodopsin masked the C-terminal monoclonal antibody epitope and resulted in the loss of rhodopsin immunoreactivity. The loss of immunoreactive rhabdomeric rhodopsin only occurred when rhodopsin was depleted from the plasma membrane such that it was found within the rhabdomere at stoichiometric levels with arrestin.When rhodopsin and arrestin are found at equal levels, binding of arrestin to rhodopsin results in the masking of the antibody epitope on the C-terminus of rhodopsin. Since masking can only occur after most of the rhodopsin has been depleted from the rhabdomere, it can be concluded that during endocytosis-induced retinal degeneration, much of the rhodopsin is localized to the cell body in small puncta. These data suggest that rhodopsin is at extremely high local concentrations in the cytoplasm. The data are discussed in the context of a model for photoreceptor cell apoptosis in retinal degenerative disorders.