The maturation and distribution of the endocytotic apparatus in outside cells of cleavage-stage mouse embryos have been studied to determine the nature and sequence of changes associated with the differentiation of the polarized trophectoderm epithelium of the blastocyst. Various quantitative and qualitative techniques used at the light and electron microscopic levels have revealed an incremental pattern of endocytotic maturation and polarization. Oocytes, eggs and blastomeres within embryos up to the early 8-cell stage contain clusters of prelysosomal endocytotic vesicles (endosomes) distributed randomly in the cortical cytoplasm. During the 8-cell stage and continuing into the early 16-cell stage, endosomes become progressively localized in the apical cytoplasm beneath the microvillous pole. Endosome polarization is initiated prior to overt polarization of the surface membrane. Concomitant with endosome polarization, pinocytotic activity at the cell surface, revealed by horseradish peroxidase labelling, becomes segregated preferentially to the apical rather than the basolateral membrane. The final maturation phase occurs at the late 16-cell stage when secondary lysosomes, characterized by trimetaphosphatase reactivity, form and polarize in the basal cytoplasm.