The kinetic behavior of cGMP-specific phosphodiesterase in a totally bleached bovine retinal rod outer segment suspension was studied by the pH-metric method at high and low concentrations of free calcium ions (≈ 100 μM and 10 nM, respectively). The phosphodiesterase was activated by low GTP concentrations (about 1-2 μM) that were comparable with the concentration of G-protein transducin, its GTP-binding alpha-subunit was the intrinsic activator of photoreceptor phosphodiesterase. The results allow the suggestion that besides the earlier described system of RGS proteins, participating in the acceleration of GTP hydrolysis, rod outer segments also contain an additional Ca(2+)-dependent mechanism to inactivate so called "free transducin", i.e. active transducin that has not managed to interact with phosphodiesterase during the time, restricted by duration of photoreceptor response.