Entry into S phase of the mitotic cell cycle is normally strictly dependent on progression through the preceding M phase. In contrast, during endoreduplication, which accompanies post-mitotic cell growth in many organisms, repeated S phases occur without intervening M phases. Upon transition from mitotic to endoreduplication cycles in Drosophila embryos, expression of the mitotic cyclins A, B and B3 is terminated and Cyclin E expression is changed from a continuous into a periodic mode [1-3]. Here, we address whether these changes in cyclin expression are required for endoreduplication by continuously expressing Cyclin A, B, B3 or E in the salivary glands of Drosophila throughout late embryonic and larval development. With the exception of Cyclin A, expression of which inhibited endoreduplication effectively but only in a few, apparently randomly distributed, cells of the salivary gland, mitotic cyclin expression was found to have no effect. In contrast, Cyclin E expression resulted in a striking inhibition of endoreduplication and growth, preceded initially by an ectopic S phase occurring just after the onset of ectopic Cyclin E expression. This observation is consistent with our previous findings that Cyclin E is required, and pulses of ectopic expression are sufficient, for triggering endoreduplication S phases . Our results indicate that Cyclin E activity, which triggers DNA replication, needs to be down-regulated to allow a subsequent S phase in vivo.