WHEN light is absorbed in the outer segment at one end of a vertebrate retinal rod or cone, sensory signals quickly reach its other end where its synapse with the visual system lies. Because rods and cones resemble short neurones in structure, it is thought that these signals travel as membrane polarization induced by light-activated current generators in the outer segments. Indeed, the membrane potentials of some rods and cones do increase in light^1–3, but it is not clear whether the effect is primary and not the result of synaptic feedback from deeper retinal neurones. Attempts to show that the outer segments are sources of membrane current by analysing the a-wave and PIII components of the electroretinogram⁴ with intraretinal microelectrodes have not yielded consistent results, however^5–8. We have therefore studied the spatial distribution of membrane current of the rods of the rat retina by an improved method using arrays of extracellular microelectrodes inserted in the receptor layer under direct vision by infrared microscopy.