This paper compares the rhythmic features obtained in natural and in synthesized speech along three dimensions: the speech type (synthesized vs natural speech), the literary genre (rhymes, poems vs story telling), and the communication setting (speech addressed to children vs addressed to adults). The study is based on the analysis of duration patterns observed in a corpus recorded by two speakers and generated by a synthesis system. Results show that rhythmic differences between synthesized and natural speech are mostly due to the marking of prosodic structure by duration.This work is achieved within a project aiming at improving the naturalness of synthesized speech for use in pedagogical software.