Absorption difference spectra for the singlet excited states of the Fenna-Matthews-Olson (FMO) complex from the green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum were simulated by exciton theory. The same assumptions and parameters were used as applied earlier (Louwe, R. J. W.; Vrieze, J.; Hoff, A. J.; Aartsma, T. J. J. Phys. Chem. B 1997, 101, 11280. Vulto, S. I. E.; de Baat, M. A.; Louwe, R. T. W.; Permentier, H. P.; Neef, T.; Miller, M.; van Amerongen, H.; Aartsma, T. J. J. Phys. Chem. B 1998, 102, 9577). The difference spectra show a bleaching near the wavelength of excitation, due to ground-state bleaching and stimulated emission. Additional negative and positive bands reflect changes in interaction with other bacteriochlorophylls than the one that is mainly excited at the transition frequency. Simulated spectra were compared with experimental difference spectra obtained by pump-probe experiments in the femto- and picosecond time region with excitation pulses in the spectral range 800-828 nm. In general, good agreement was obtained. Various difference spectra developed during the first 1 to 2 ps, but in all cases the system relaxed to the lowest energy state, which was largely completed in 10 ps.