The mechanism by which bacteriorhodopsin is activated following light absorption is not completely clear. We have detected protein conformational alterations following light absorption by retinal-based chromophores in the bacteriorhodopsin binding site by monitoring the rate of reduction-oxidation reactions of covalently attached spin labels, using EPR spectroscopy. It was found that the reduction reaction with hydrolxylamine is light-catalyzed in the A103C-labeled pigment but not in E74C or M163C. The reaction is light- catalyzed even when isomerization of the C13=C14 bond of the retinal chromophore is prevented. The reverse oxidation reaction with molecular oxygen is effective only in apomembrane derived from the mutant A103C. This reaction is light-accelerated following light absorption of the retinal oxime, which occupies the binding site. The light-induced acceleration is evident also in 'locked' bacteriorhodopsin in which isomerization around the C13=C14 bond is prevented. It is evident that the chromophore-protein covalent bond is not a prerequisite for protein response. In contrast to the case of the retinal oxime, a reduced C=N bond A103C-labeled pigment did not exhibit acceleration of the oxidation reaction following light absorption. Acceleration was observed, however, following substitution of the polyene by groups that modify the excited stated charge delocalization. It is suggested that protein conformational alterations are induced by charge redistribution along the retinal polyene following light absorption.