We propose to synthesize and incorporate into bovine rhodopsin, 13C-retinals labelled at specific positions in the ring or polyene chain. The 13C chemical shifts for the labelled retinal as the visual pigment chromophore will be determined by nuclear magnetic resonance spectroscopy. This will allow determination of the charge density at the labelled carbon position. When a sufficient number of positions have been evaluated the information obtained will be utilized as an imput parameter for molecular orbital calculation aimed at elucidation of the ground state structure of the chromophoric retinal. This should allow us to contribute valuable information toward the understanding of "why visual pigments are colored".