Changes in chlorophyll fluorescence yield are related to changes in the photosynthetic efficiency of plants. Under normal conditions, approximately 97% of the light energy absorbed by chlorophyll is converted to oxidation and reduction potential to drive electron transport in photosynthesis. Stress conditions such as those caused by toxic chemicals can reduce the efficiency of photosynthesis by disturbing the pigment protein apparatus or blocking the electron transport system in the chloroplast. The 6 to 10% loss of absorbed light energy due to stress conditions appears as chlorophyll fluorescence. A chlorophyll fluorescence assay can provide a simple, rapid method for measuring the effect of physical and chemical stress on the photosynthetic efficiency of plants. In this study, seedlings of soybean (Glycine max L. Merr.) and barley (Hordeum vulgare L.) were analyzed by chlorophyll fluorescence induction following treatment with three herbicides dissolved in water at three concentrations. The eluate of soil collected from a hazardous landfill was also tested. Several endpoints were measured; initial fluorescence (F0), variable fluorescence (Fv), maximum fluorescence (Fm), Fv/Fm ratio, and electron pool (EP). This study indicated that the decline of photosynthetic efficiency of plants exposed to toxic chemicals in water or soil eluate can be measured by the chlorophyll fluorescence assay method.