In order to examine the possibility that cyclic-GMP is an internal transmitter in the process of phototransduction, cyclic-GMP was iontophoretically injected into a gecko photoreceptor from one barrel of a double-barrelled microelectrode with the other barrel monitoring membrane potential and conductance changes. Cyclic-GMP induced a prolonged depolarization accompanied by a marked increase in membrane conductance. In the sodium-free bathing solution (substituted by choline), injection of cyclic-GMP did not cause any depolarization. The cells, which showed the cyclic-GMP-induced depolarization, produced no significant potential changes with injection of 5′-GMP or cyclic-AMP, indicating that the depolarization was cyclic-GMP-specific. The results suggest that cyclic-GMP is involved in the phototransduction process in the gecko photoreceptors. However, uncertainty still remained as to whether the cyclic-GMP level directly correlates with the photoreceptor membrane potential.