This review examines the ionic conductance that generates the electrical response to light in rods and cones of the vertebrate retina. In visual physiology this conductance is commonly referred to as the light-regulated or light-sensitive conductance. The designation cGMP-activated con ductance is perhaps preferable because it is now known to be controlled by guanosine 3',5'-cyclic monophosphate (abbreviated here as cGMP); light modulates the conductance solely by changing the intracellular con centration of cGMP. We shall use the different terms for the conductance interchangeably. A review on this channel seems timely in view of the speed of recent progress in understanding its nature and function. The conductance is interesting for several reasons. First, it has physio logical properties that are beautifully suited for its role in visual trans duction. Second, it is the first ionic channel found whose gating is directly controlled by a cyclic nucleotide. Third, there are indications that the conductance may belong to a new family of ion channels involved in signal