The absorption of light by the chromophore of rhodopsin initiates a series of interconversions between spectrally distinct intermediates. The possibility has been raised that one of these transitions is accompanied by a change in the state of rhodopsin, and that it is this change which instigates visual excitation via a cascade of enzyme catalysed reactions. It has been suggested that the initial step of this cascade, which leads to the activation of cyclic GMP phosphodiesterase (PDE), involves the interaction of a GTP-binding regulatory (G) protein with rhodopsin. The ability of rhodopsin to activate PDE may be inhibited by the phosphorylation of sites exposed on the opsin surface as a result of light-induced conformational changes. To obtain more information about the relationship between the postchemical and biochemical reactions of rhodopsin we have investigated which transition leads to the activation of rhodopsin as a substrate for rhodopsin kinase, and report here that it is the transition from lumirhodopsin to metarhodopsin I.