Trinitrotoluene, as a compound of conventional explosive, may cause inhibitory effect on terrestrial plants. When Lactuca sativa was exposed to different concentrations of trinitrotoluene (32-1000 mg/kg), photosynthetic process was investigated by using rapid chlorophyll fluorescence kinetic and pulse modulated fluorometry. The decrease of chlorophyll a variable fluorescence was seen to be caused by the deactivation of photosystem II reaction centers. We found for rapid variable fluorescence to be a useful indicator to evaluate the inhibitory effect of trinitrotoluene on photosystem II primary photochemistry and electron transport. The fluorescence parameters, related to the reduction state of photosystem II and to non-photochemical dissipation of light energy, showed a strong relation between the inhibitory effect of photosystem II activity and concentration of trinitrotoluene. The change of photosynthetic fluorescence parameters induced by trinitrotoluene was a reliable indication of the plant physiological state. We proposed for the reduction state of photosystem II and the non-photochemical energy dissipation to be a useful tool in bioassay toxicity testing of trinitrotoluene polluted soil.