Transduction in vertebrate photoreceptors takes place via an enzymatic cascade in which photoisomerised photopigment acts via a G-protein, transducin, to activate a phosphodiesterase enzyme which hydrolyses cGMP, leading to the closure of cyclic nucleotide-gated channels in the outer segment membrane and the electrical response to light. The time constant which dominates light response recovery is set by the slowest of the steps that quench the phototransduction cascade. Candidates for this dominant time constant comprise either the shutoff of photopigment catalytic activity or shutoff of the activated transducin-phosphodiesterase complex. This distinction is functionally important, since photopigment quenching, but not transducin shutoff, is known to be modulated by the decline in Ca2+ concentration which takes place during the light response. In rod photoreceptors, the dominant time constant for response recovery is not affected by Ca2+, thereby excluding rhodopsin quenching. Instead, it has been shown to depend on the GTPase activity of transducin, which therefore dominates response recovery. However, in cones of all colour classes the time constant that dominates response recovery depends strongly on Ca2+, and can be modulated by manipulations that are known to affect photopigment quenching. These observations indicate that in cones, unlike rods, photopigment quenching dominates response recovery and provides an additional mechanism for adaptation of the cone response during steady light.