(1) The kinetics of the change in intensity of 1-ms delayed fluorescence following the onset of illumination have been studied in chromatophores of Rhodopseudomonas capsulata and compared with the kinetics of the light-induced carotenoid shift, and of the change in fluorescence yield. The change in intensity of delayed fluorescence, when plotted on a logarithmic scale, followed closely the kinetics of carotenoid change, but not the change in fluorescence yield. (2) The antibiotics valinomycin, nigericin and antimycin, showed qualitatively similar effects on delayed fluorescence and the carotenoid shift, but the effects on fluorescence yield were quite dissimilar. (3) The extent of the carotenoid shift, induced by KCl pulses in the presence of valinomycin was used to calibrate the change as an indicator of membrane potential. The action of nigericin on both carotenoid shift and delayed fluorescence was used to relate the two, and a linear relationship was demonstrated between the logarithm of the intensity of the delayed fluorescence and the extent of the carotenoid shift. This relationship indicated that the intensity of delayed fluorescence was proportional to the exponential of the membrane potential. (4) It is suggested that the electrical component of the transmembrane electrochemical H+ gradient directly lowers the activation energy for emission, but that the pH component has no such effect.