Summary— A cytological analysis was performed in order to determine how the formation of the metaphase I‐ and metaphase II‐spindles is dependent upon p34cdc2 kinase activity and protein synthesis during the meiotic maturation of Xenopus oocytes. The p34cdc2 kinase activity increases abruptly during the prophase‐prometaphase I transition, then drops to a minimum level at the metaphase I/anaphase I transition and further increases again until reaching a maximum stable level at metaphase II. The injection of an indestructible cyclin B into oocytes arrests the maturation process at the onset of anaphase I and prevents the re‐increase of p34cdc2 activity which accompanies normal entry into metaphase II. Inhibition of protein synthesis between the germinal vesicle breakdown and the onset of metaphase I spindle induces exit from M‐phase and leads to an ‘interphase‐like’ state characterized by the organization of nuclear‐like structures. In contrast, inhibition of protein synthesis at metaphase II stage does not affect the metaphase II spindle nor p34cdc2 activity, indicating that metaphase I‐ and metaphase II‐spindles are not regulated by the same effectors. When protein synthesis is inhibited before induction of M‐phase by MPF transfer, it prevents the formation of the metaphase I spindle, despite a transient elevated level of p34cdc2 activity. To dissociate the role of protein synthesis and of p34cdc2 kinase activity, the indestructible cyclin B was microinjected in the absence of protein synthesis. This allows the in vivo maintenance of a stable p34cdc2 activity. Under these conditions, microtubules do not polymerize and no metaphase I spindle is formed, demonstrating that the synthesis of proteins, different from cyclin B, is required for the organization of the metaphase I spindle in addition to the presence of p34cdc2 activity.