Lipophilic material produced by buds and young leaves of Populus nigra L. contains flavonoid aglycones. Under precise experimental conditions a green-yellow fluorescent light can be detected from these phenolic compounds. A cytophotometric technique using this property has been established: localisation and quantitative estimation of intracellular flavonoid material are realized by fluorometry. The accumulation and localisation of flavonoids in the different cellular compartments (plasts, vacuoles, cytoplasm, cell walls) have been determined for each period of the secretory cycle. These results were obtained from, in situ, investigations or from experiments with ethanolic extracts of cellular compartments.