The mechanism by which a surface pole of microvilli is generated in mouse 8-cell blastomeres has been investigated. 4-cell and 8-cell embryos (or cell couplets) were incubated for precise times during their respective cell cycles in medium containing cytochalasin D (CCD) to disrupt the microfilament system. The blastomeres were analysed immediately for the distribution and state of organization of their microvilli, using three morphological techniques. The results indicate that the surface pole, characterized by microvilli containing CCD-resistant core filaments, is not generated by the gradual segregation of stable microvilli to the apical surface. An alternative model is proposed, based upon (a) the stabilization of the apical cytocortex prior to the elongation of apical microvilli and (b) the destabilization of cytocortical elements in contact areas and the shortening and loss of basolateral microvilli.