IT has been suggested from spectroscopic observations of rhodopsin1–3 that the photoisomerization of chromophore—11-cis to all-trans retinal—must be followed by a conformational change in the protein moiety of the rhodopsin molecule, which is considered to play an important part in the initiation of visual impulses. Such a conformational change in the protein should be determined by direct measurement of the optical rotation, as this is believed to be closely related to the conformation of the protein4–6. The purpose of the present investigation was to determine how far the rotatory polarization of rhodopsin solution changes in response to bleaching by light.