Nordidemnin (NorD) is a cyclic depsipeptide isolated from a Caribbean tunicate. This drug is thought to affect cell proliferation by acting on protein, RNA, and DNA syntheses. We studied the ability of NorD to arrest sea urchin embryos at the prophase stage. We tested whether NorD could alter the synthesis and phosphorylation of proteins as well as polyphosphoinositide (PPI) metabolism, as the activation of these processes is necessary for progression through the cell cycle. The dose-response effect on protein synthesis and cell cleavage suggests that NorD acts in the same way as emetine, a well-known protein synthesis inhibitor. We observed that treatment of eggs with emetine or NorD, even at concentrations that inhibited DNA and protein synthesis as well as phosphorylation of proteins, led to modifications in the incorporation of 32P into phosphatidylinositol phosphate and phosphatidylinositol bisphosphate without any alteration in the chemical amounts of these lipids. However, fluctuations in the PPI messenger system that occur during the cell cycle were maintained in the presence of either drug. We suggest that proteins and PPI are linked in an intricate network to control mitosis and cellular proliferation.