The enzyme soluble guanylyl cyclase (SGC) mediates physiological effects of the gaseous signalling molecule nitric oxide by generating the second messenger molecule cyclic–GMP (cGMP). Here we have demonstrated that SGC is expressed in photoreceptor cells of locust compound eyes. However, stimulation of SGC activity in the eyes was observed only in the dark, indicating that light may cause inhibition of SGC activity in locust photoreceptor cells. Because light causes elevation of cytosolic Ca2+ in insect photoreceptor cells, we investigated the involvement of Ca2+ in mediating the inhibitory effect of light on SGC activity in the locust eye. Light–adapted locust eyes incubated with Ca2+–free physiological saline displayed a similar level of stimulated SGC activity to that normally seen only in dark–adapted eyes. These data indicate for the first time that Ca2+ may regulate SGC activity in cells. Moreover, the dark dependence of SGC activity in the locust eye suggests that SGC and cGMP may participate in dark–adaptation mechanisms in insect photoreceptor cells.