The light-induced transient absorbance changes which are affected by valinomycin have been studied using magnetically oriented spinach chloroplasts and a polarized measuring beam. The delta A spectra for the two polarizations parallel and perpendicular to the plane of the photosynthetic membranes have been recorded in the spectral range 630-750 nm. Large polarization effects are found in all the bands of the delta A spectrum, shifts in the position of the extrema are observed and the two features with available data on the dichroism of the Stark effect on monomolecular hypothesis of the electrochromic nature of these absorbance changes in vivo. The data on this electrochromic effect can be correlated with the linear dichorism of oriented chloroplasts and the (see article) spectrum in the 645-655 nm region gives further evidence of the orientation out of the membrane plane of the red transition moment of chlorophyll b.