Phototransduction in the Drosophila retina appears to require the phosphoinositide signaling cascade following receptor/G-protein activation. Subsequent opening of membrane cationic channels causes excitation. The biochemical events underlying channel opening and regulation of sensitivity remain largely unknown. Evidence is mounting that phototransduction in Drosophila and other invertebrate species may additionally involve the second messenger, cyclic-GMP (cGMP). We report that exogenous cGMP influenced Drosophila retinal phototransduction in two ways. In whole cell tight-seal voltage-clamp experiments, membrane permeant cGMP analog, 8-bromo-cyclic-GMP (8-Br-cGMP), induced membrane currents and dramatically enhanced light-induced currents. The currents induced by 8-Br-cGMP possessed reversal potentials similar to those induced by light. The magnitudes of cGMP-induced currents exhibited marked dependence on intensity of background illumination. Potential direct or modulatory roles of cGMP in Drosophila phototransduction are discussed.