The rod-driven, All amacrine cells in the mammalian retina maintain homologous gap junctions with one another as well as heterologous gap junctions with on-cone bipolar cells. We used background illumination to study whether changes in the adaptational state of the retina affected the permeabilities of these two sets of gap junctions. To access changes in permeability, we injected single All amacrine cells with the biotinylated tracer, Neurobiotin, and measured the extent of tracer coupling to neighboring All cells and neighboring cone bipolar cells. We also measured the center-receptive field size of All cells to assess concomitant changes in electrical coupling. Our results indicate that in well dark-adapted retinas, All cells form relatively small networks averaging 20 amacrine cells and covering about 75 f«n. The size of these networks matched closely to the size of All cell on-center receptive fields. However, over most of their operating range, All cells formed dramatically larger networks, averaging 326 amacrine cells, which corresponded to an increased receptive-field size. As the retina was light adapted beyond the operating range of the All cells, they uncoupled to form networks comparable in size to those seen in well dark-adapted retinas. Our results, then, indicate that the adaptational state of the retina has a profound effect on the extent of electrical coupling between All amacrine cells. Although we observed light-induced changes in the number of tracer-coupled cone bipolar cells, these appeared to be an epiphenomenon of changes in homologous coupling between All amacrine cells. Therefore, in contrast to the robust changes in AII-AII coupling produced by background illumination, our data provided no evidence of a light-induced modulation of coupling between All cells and on-cone bipolar cells.