A model is described having the following features: Light induces Ca2+ release from vertebrate rod outer segments discs via pores composed of multimeric rhodopsin. Cytoplasmic Ca2+ reversibly blocks Na+ channels of the surface membrane, with the time course of development and amplitude of the response to light being influenced by restrictions on intradiscal Ca2+ diffusion. The falling phase of response reflects a decline in cytoplasmic [Ca2+] due to a Ca2+-binding protein controlled by cyclic GMP so that its binding capacity is increased by the reduction in cytoplasmic [cyclic GMP] which follows rhodopsin bleaching.