Cyclic nucleotide-gated (CNG) ion channels mediate phototransduction in photoreceptors and chemotransduction in olfactory cells. The binding of up to four ligands activates CNG channels by an allosteric mechanism which is not yet fully understood. We expressed CNGA1 channels in Xenopus oocytes and studied channel activation by photolysis-induced jumps of the cGMP concentration and by voltage steps. As caged cGMP for flash photolysis, the [7-(diethylamino)coumarin-4-yl]methyl ester of cGMP (DEACMcGMP), the [6,7-bis(carboxymethoxy) coumarin-4-yl]methyl ester of cGMP (BCMCMcGMP), and the {7-[bis(carboxymethyl)amino]coumarin-4-yl}methyl ester of cGMP (BCMACMcGMP) were used. We show that the activation gating following cGMP jumps is largely similar to the activation and deactivation gating following voltage steps at the same cGMP concentration and it is maximally slow at concentrations near the EC₅₀ value. cGMP jumps to high concentrations caused openings of CNGA1 single-channels directly to the level observed at saturating cGMP without passing sub- or superlevels. Permeating ions passing the pore more slowly (Rb⁺>K⁺ >Na⁺) slow the activation time course. We conclude that the CNGA1 gating is not rate limited by the cGMP binding but by an allosteric reaction and that conformational changes in the pore essentially contribute to this allosteric reaction.