Previous works have shown a decrease in sunflower net assimilation during its cycle of development, but without information on the main causes. A pot experiment demonstrated the small effect of the source-sink relationships on the time-course of photosynthesis in sunflower ; in this plant, assimilates can be stored either in the vegetative organs or in the seeds. A parallel field experiment set up to obtain different plant densities after the establishment of the vegetative apparatus, tried to determine the effect of light received at different levels of the canopy on assimilation. While photosynthetic level was low in the lower levels of the dense population (10 plants per square metre), it remained high in the absence of intraspecific competition (1 plant per square metre), leading to a doubling of total dry matter production per plant. Ageing and reduction of the photosynthetic activity of the lower leaves was linked to the light level received and to their physiological status. As self-shading increased, we noticed a marked reduction in the total nitrogen content of the shaded leaves ; the most sensitive protein fraction could be the heavy one, containing RDP carboxylase. Thus, while photosynthesis during the grain filling stage did limit yield, photosynthetic activity was more affected by environmental conditions and perhaps by plant nutrition than by source-sink relationships