1. In dogfish, studies were made of electrical connexions between pairs of cells giving S-potentials. After recording, the types and locations of the two cells were determined morphologically by electrophoretically injecting two different fluorescent dyes.2. Hyperpolarizing S-potentials were observed in external and internal horizontal cells; depolarizing responses were seen in bipolar cells. All pairs of external horizontal cells examined were electrically coupled, i.e. intra-cellular polarization of one horizontal cell gave rise to polarization in its neighbour. Coupling was detected for cells separated by five other cells. The electrical coupling was non-rectifying. No coupling was found between external and internal horizontal cells or between external horizontal cells and bipolar cells.3. Generally the smaller the distance between cells, the larger the coupling ratio, but some pairs of cells separated by similar distances showed widely different coupling ratios.4. The coupling between cells was also demonstrated by the finding that Procion Yellow injected into one external horizontal cell diffused into one or two neighbouring external horizontal cells. It did not diffuse into internal horizontal cells or bipolar cells.5. Close membrane apposition has been reported between neighbouring external horizontal cells, but not between these and either bipolar or internal horizontal cells. The distribution of electrical connexions observed in the present study agrees with these anatomical findings.