We have examined the hypothesis that Ca(2+)-dependent cyclic-GMP metabolism may play a role in visual transduction in Limulus photoreceptors. Although phosphoinositide hydrolysis is central to phototransduction and phosphoinositide-dependent Ca(2+)-mobilization seems to be required for transduction, the subsequent steps leading to ion channel gating (the immediate cause of excitation) are not understood. Channels normally opened in response to light can be opened in excised membrane patches by cGMP but not by Ca2+, suggesting that cGMP acts as a channel ligand in excitation. Using phosphodiesterase inhibitors, we investigated whether changes in cGMP metabolism could affect excitation. We report that zaprinast and IBMX increased the amplitudes and retarded the kinetics of physiological light responses. These effects were maximal for brightest stimuli. The effects were markedly enhanced in low Ca2+ conditions. In contrast, excitation induced by direct IP3-injection and by direct Ca(2+)-injection were inhibited. These observations suggest that PI-induced excitation is dependent on cGMP metabolism in a Ca(2+)-dependent manner, and they support the possibility that transduction involves modification of cGMP metabolism by Ca(2+)-release resulting from phosphoinositide hydrolysis.