The spectrally and temporally resolved fluorescence properties of native bacteriorhodopsin (bR) and bR reconstituted with a nonisomerizing analog of the retinal Schiff base (bR5.12) are examd. The first attempt to exptl. monitor the excited state relaxation processes in both type of pigments using ultrafast fluorescence spectroscopy is reported. The fluorescence is emitted from retinal mols. in an all-trans configuration. Substantial energy relaxation involves very fast intramol. and intermol. vibrational modes and these are shown to occur on a time scale faster than isomerization. The possible contribution of dielec. interaction between the retinal Schiff base and the protein environment for the excited state energy relaxation is discussed. [on SciFinder (R)]