This paper describes an easy experiment to study the absorption and action spectrum of photosynthesis, as well as the inhibition by heat, high light intensity and the presence of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) on the photosynthetic process. The method involves measuring the chlorophyll fluorescence emitted by intact leaves of oat (Avena sativa, var. Prevision) plants. Measuring the variable fluorescence emission in leaves illuminated by monochromatic light (blue, green, red and far red), an action spectrum of photosynthesis was obtained, showing that blue and red lights are the most effective to produce photosynthesis. The quantum yield of the photosystem (PS) II and the variable fluorescence values in the leaves decreased by treatments with white light of high intensity and with heat (up to 35 degrees C), indicating inhibition of the PS II. A synergistic effect of both stress factors on photosynthesis inhibition was also observed. Incubation with DCMU of the leaves also decreased the quantum yield of PS II and the variable fluorescence emission, as a consequence of the inhibition of PS II by DCMU. This practical is intended for students of plant biochemistry and plant physiology. (Contains 1 table and 6 figures.)