The direct linkage between the bleaching of the visual pigment rhodopsin and the control of such visual processes as excitation and adaptation is not known. Hence, it has been difficult to design a good biochemical/physiological test for the ability of modified visual pigments to substitute for native pigments. One criterion is to see if non-physiological pigments lose their color after irradiation, ie, bleach. However, this test is inadequate because the bleaching of the pigment does not insure the ability to initiate a physiological event. Another approach is the creation of non-physiological pigments in the retina by regeneration of bleached opsin in situ with exogeneously applied chromophores [1, 2]. With this method, one can ask if the pigments formed can help to restore the threshold of receptor potential, which was raised by the bleaching. Although this technique is a very promising one, it is not clear if all modified chromophores could pass across the rod plasma membrane so as to form the pigment in situ and the experiments are restricted to a single type of the opsin. A new approach to test for the ability of non-physiological pigments to evoke the triggering action of bleached rhodopsin has been opened up by the discovery that light can mediate the activity of enzymes in the retina. These enzymes, a phosphodiesterase and a GTPase operating in close conjuction [3-51, can control cyclic nucleotide concentrations. Exactly what cyclic nucleotides do in vision is still not understood. Nevertheless, there is widespread agreement that they have an important role because of the enormous biochemical machinery that has been devoted to the control of nucleotide concentrations in photoreceptor cells and because it is light that controls these levels [4]. The presence of these light-activated enzymes provides the possibility for a simple test to see if any artificial or non-physiological rhodopsin is physiologically active, ie, can light absorbed by the pigment lead to the activation of the phosphodiesterase?