The binding of a ligand to a receptor causes a conformational change on the receptor which can trigger a biological signal. To understand the molecular mechanism of the receptor action, it is important to measure ligand binding and conformational change at the same time, because, according to the principle of reciprocity, binding and conformational change interdepend on each other. We synthesized a fluorescent cGMP derivative, fcGMP. FcGMP activates CNGA2 channels similar as does cGMP. This allowed us to simultaneously read out channel activation and the degree of ligand binding. The binding of fcGMP was quantified in the dome of inside-out patches using a confocal microscope. The fluorescence of the bound fraction of the ligand was identified by means of a second dye, DY647, which did not bind to the channels but labelled the bath solution. We show that at low [fcGMP] binding exceeds activation whereas at higher [fcGMP] activation exceeds binding, resulting in a crossover of the relationships. In combination with activation time courses induced by [cGMP] jumps, we favour a kinetic model with four binding steps of which only two suffice to exert full channel activation.