Lateral interactions are important for the processing of visual information by the retina. In the proximal retinal information is passed from amacrine cell to amacrine cell to influence the receptive fields of bipolar cells and ganglion cells. The horizontal cells are extensively coupled through electrical junctions. Large areas of gap junctional contact between horizontal cells has been demonstrated in the electron microscope. This chapter investigates the properties of coupling between the cells, particularly the mechanisms underlying the modulation of gap junctional conductance by dopamine. To determine the effects of dopamine on horizontal cell gap junctional conductance, dopamine was applied to pairs of cells by pressure ejection from a micropipette containing dopamine dissolved in Ringer's. Dopamine was found to dramatically alter coupling between pairs of isolated horizontal cells. The action of Cyclic AMP (cAMP) to decrease horizontal cell junctional conductance could be mediated through several possible mechanisms.