A binding colchicine assay together with an immunostaining study with an anti-tubulin antibody showed that taxol, when added to the incubation medium, induces the formation of new microtubules in the Xenopus oocyte cortex. The capacity of the tubulin assembly in the submembranous cytoplasm decreases in progesterone-matured oocytes. In contrast, in enucleated matured oocytes this change does not occur. Altogether, these results show that taxol provokes tubulin assembly exclusively in the cortex of prophase oocyte, whereas in normal matured oocytes both cortical and cytoplasmic cytaster microtubules can be induced by taxol. The swelling of the oocyte nucleus therefore controls the spatial distribution of nucleation centers for tubulin assembly during meiotic maturation.