This review examines the roles of guanosine 3' ,5' -cyclic monophosphate (cGMP) and Ca2+ ions in signal transduction of vertebrate photoreceptors. Rod and cone photoreceptor cells transduce the absorption of light into a brief voltage pulse by closing cation channels in the plasma membrane. In recent years there have been many striking advances in understanding the physico chemical basis of photo-electrical excitation in photoreceptor cells. An in tracellular messenger, cGMP, carries the message of the absorption event to ion channels in the surface membrane of the photoreceptor outer segment. Light decreases the cGMP concentration by activating an enzyme cascade that hydrolytic ally destroys the intracellular messenger. A negative feedback loop involving Ca2+ and a guanylyl cyclase (GC) controls the recovery of the photoreceptor cell from the light response. A similar transduction scheme with cAMP as the intracellular messenger exists in sensory neurons of the olfactory epithelium (73). Light also initiates a sequence of events that changes the sensitivity and response kinetics of photoreceptors. These cellular processes-collectively called adaptation-enable the photoreceptor cell to adjust its sensitivity to the ambient illumination. Although we know much less about adaptation, recent experiments suggest that Ca2+ ions play an important role by controlling the formation of cGMP.