We studied the cellular mechanisms underlying the induction of polarity in individual blastomeres of the 8-cell mouse embryo. The ability to induce polarity is lacking in the membranes of unfertilized and newly fertilized mouse eggs, then develops during the 2-cell stage, and is present in membranes of cells from 4-, 8-, and 16-cell stages. The axis of polarity takes 3-5 h to become established and thereafter appears to be stable. Multiple cell contacts affect the orientation of the axis of polarity, and no polarity develops in cells which are totally surrounded. Polarized cells show evidence of an limited capacity for slight adjustments in their position relative to other cells. The implications of these results for the mechanisms by which a blastocyst is generated are discussed briefly.