Stimulation of the retina by light leads to a decrease in the inward current that normally flows into rod outer segments in the dark. Both cGMP and Ca2+ have been proposed as internal messengers that mediate the response and adaptation to illumination by regulating this 'light-sensitive' inward current. Since several lines of evidence suggest that the levels of cGMP and Ca2+ change with photostimulation, it was of interest to determine if these substances affect the light-sensitive current in rods. In the present study, solitary cultured photoreceptors were voltage-clamped and treated with TEA, Cs+ and Co2+ to isolate the light-sensitive current. Using this new method, intracellular injection of either cGMP or EGTA produced an increase in the inward current in the dark. Subsequent responses to bright flashes were larger, probably because there was more light-sensitive current to be suppressed. With longer injections of these substances, increasingly brighter flashes were necessary to elicit a maximum response. Since the reversal potential of the cGMP- or EGTA-induced current was the same as the normal light-sensitive current, the ionic basis of the two currents appeared identical. The finding that a change in Ca2+ or cGMP can modulate the light-sensitive current supports the notion that both may act as internal messengers in rod outer segments.