Light absorption by the visual pigment rhodopsin leads to vision via a complex signal transduction pathway that is initiated by the ultrafast and highly efficient photoreaction of its chromophore, the retinal protonated Schiff base (RPSB). Here, we investigate this reaction in real time by means of unrestrained mol. dynamics simulations of the protein in a membrane mimetic environment, treating the chromophore at the d. functional theory level. We demonstrate that a highly strained all-trans RPSB is formed starting from the 11-cis configuration (dark state) within .apprx.100 fs by a minor rearrangement of the nuclei under preservation of the saltbridge with Glu113 and virtually no deformation of the binding pocket. Hence, the initial step of vision can be understood as the compression of a mol. spring by a minor change of the nuclear coordinates. This spring can then release its strain by altering the protein environment. [on SciFinder (R)]