In frog photoreceptor membranes, light induces a dephosphorylation of two small proteins and a phosphorylation of rhodopsin . The level of phosphorylation of the two small proteins is influenced by cyclic GMP . Measurement of their phosphorylation as a function of cyclic GMP concentration shows fivefold stimulation as cyclic GMP is increased from 10 -5 to 10-3 M . This includes the concentration range over which light activation of a cyclic GMP phosphodiesterase causes cyclic GMP levels to fall in vivo . Cyclic AMP does not affect the phosphorylations . Calcium ions inhibit the phosphorylation reactions . Calcium inhibits the cyclic GMP-stimulated phosphorylation of the small proteins as its concentration is increased from 10 -s to 10 -3 M, with maximal inhibition of 70% being observed . Rhodopsin phosphorylation is not stimulated by cyclic nucleotides, but is inhibited by calcium, with 50% inhibition being observed as the Ca" concentration is increased from 10-9 to 10-3 M . A nucleotide binding site appears to regulate rhodopsin phosphorylation . Several properties of the rhodopsin phosphorylation suggest that it does not play a role in a rapid ATP-dependent regulation of the cyclic GMP pathway . Calcium inhibition of protein phosphorylation is a distinctive feature of this system, and it is suggested that Ca" regulation of protein phosphorylation plays a role in the visual adaptation process . Furthermore, the data provide support for the idea that calcium and cyclic GMP pathways interact in regulating the lightsensitive conductance .