Using intense ns-laser pulses up to 2 x 10** photons per cm^ per pulse the intensity dependence of fluorescence yield (O) of different photosystem 2 (PS2) subunits in spinách on the intensity of excitation pulse (/) was measured. A sti ong drop of O was observed with increasing laser pulse I. Measurements of the light-harvesting complex of PS2, LHC2, were taken at excitation wavelengths of 645-700 nm. A shift of the í) vs. I curves to lower I with increasing absorption cross section was observed. Measurements of PS2 and thylakoid membrane ífagments taken at a fixed excitation wavelength of 645 nm showed the same O V5, I dependences as LHC2. This suggests that LHC2-pigment proteins dominate the exciton concentration in the photosynthetic apparatus. A smáli drop of O measured at CP29 and CC2 particles was possibly caused by smaller domains. Smaller drop of O measured at D1D2 particles was similar to that of free chlorophyll (Chl), indicating that bimolecular annihilation was absent. Experimental results could be described using a simple two-level model including exciton-exciton-annihilation and depletion of Chl ground States. Good fits of measured O-intensity ciu-ves were possible for LHC2 with all the ušed excitation wavelengths. Outputs of these approximations are statements about excited statě concentration and time dependence of their decay as a function of excitation irradiance.