6-dimethylaminopurine (6-DMAP), a potent protein kinase inhibitor, drives most cells into an interphasic stage. Experiments were undertaken with oocytes from three marine invertebrate species, i.e., Mytilus edulis, Spisula solidissima, and Strongylocentrotus droebachiensis, wherein oocytes were arrested at different phases of meiosis. 6-DMAP induced a continuous DNA synthesis in meiotic cells, whereas it allowed a single round of DNA replication in treated mitotic cells, regardless of species considered. The effects of 6-DMAP were accompanied in all cases by rephosphorylation on tyrosine of the p34cdc2 homolog, the M-phase promoting factor (MPF) catalytic subunit. The fact that 6-DMAP overcomes the inhibitory control of replication during meiosis suggests that this process depends upon protein phosphorylation, while DNA synthesis regulation in mitotic cells relies on 6-DMAP-insensitive events.