The effects of Ca²⁺, Mg²⁺ and Mn²⁺ on light-evoked electrical responses, ERG and action potentials of optic nerve fibers, were studied in an isolated eye of Lymnaea stagnalis. Signals of both types persisted with minor changes when Mg²⁺ concentration in the bath solution was increased up to 15 mM. This finding suggests that action potentials of photoreceptor cells are transmitted via their axons directly to central ganglia without a relay in chemical synapses on interneurons. At the same time, the changed spiking pattern may indicate chemical interactions between photoreceptors cells themselves. In the presence of 4 mM EDTA or 10 mM Mn²⁺, both ERG waves and spiking activity were considerably depressed, probably, as a result of impaired phototransduction in photoreceptor cells.