Calcium is thought to be essential for adaptation of sensory receptor cells. However, the transduction cascade of hyperpolarizing, ciliary photoreceptors of the scallop does not use IP3-mediated Ca release, and the light-sensitive conductance is not measurably permeable to Ca2+. Therefore, two typical mechanisms that couple the light response to [Ca]i changes seem to be lacking in these photoreceptors. Using fluorescent indicators, we determined that, unlike in their microvillar counterparts, photostimulation of ciliary cells under voltage clamp indeed evokes no detectable change in cytosolic Ca. Notwithstanding, these cells exhibit all of the hallmarks of light adaptation, including response range compression, sensitivity shift, and photoresponse acceleration. A possible mediator of Ca-independent sensory adaptation is cGMP, the second messenger that regulates the light-sensitive conductance; cGMP and 8-bromo cGMP not only activate light-dependent K channels but also reduce the amplitude of the light response to an extent greatly in excess of that expected from simple occlusion between light and chemical stimulation. In addition, these substances accelerate the time course of the photocurrent. Tests with pharmacological antagonists suggest that protein kinase G may be a downstream effector that controls, in part, the cGMP-triggered changes in photoresponse properties during light adaptation. However, additional messengers are likely to be implicated, especially in the regulation of response kinetics. These observations suggest a novel feedback inhibition pathway for signaling sensory adaptation.