This study deals with the analysis of the relationships between stress and duration in French. The duration of stressed and unstressed syllables, taken from 55 read sentences, has been measured. The results have lead to conclude that, while stress is closely correlated to a modification of duration, the nature of this modification is more complex than expected, according to classical theories. The stressed syllable can be lengthened or shortened with regard to the nearest unstressed syllable and also to the other stressed syllable of the sentence. This choice seems to be allowed and motivated according to perception laws and mechanisms of expressivity.