Photoreceptors detect light through a seven-helix receptor (rhodopsin) and heterotrimeric G protein (transducin) coupled to a cyclic GMP phosphodiesterase(1,2). Similar pathways are used to amplify responses to hormones, taste and smell(3-5). The amplification of phototransduction is reduced by a fall in cytoplasmic Ca2+ (refs 6-10), but it is not known how the deactivation of rhodopsin and transducin influence this response and hence the extent and duration of phosphodiesterase activity(11-14). Here we investigate this by recording the electrical response to flashes of light in truncated rod photoreceptors(10). By removing ATP to block the deactivation of rhodopsin by phosphorylation(15), we show that this reaction limits the amplitude of the response and begins within 3.2 s of a flash in a solution containing 1 mu M Ca2+, falling to 0.9 s in a zero-Ca2+ solution. In contrast, the activation and amplitude of the response were unaffected when transducin deactivation by GTP hydrolysis was blocked by replacing GTP with its nonhydrolysable analogue GTP-gamma S-11, demonstrating that there is little GTP hydrolysis occurring over the period in which photoexcited rhodopsin is quenched. The rapid deactivation of rhodopsin is therefore a Ca2+-sensitive step controlling the amplitude of the light response, whereas transducin deactivation is slower and controls recovery.