The cleavage behavior of cells isolated from 1- to 8-cell-stage mouse embryos was studied with time-lapse video equipment; changes in cellular dimensions and their timing were recorded. The division of an isolated cell results in the formation of a twin-cell pair. The divisions of these two cells were always asynchronous. In each division the volume of a daughter cell was approximately half of that of the parental cell but its apparent surface area was 59-65% of that of the parental cell. Consequently, the ratio of apparent surface area to volume increased in each division by 25-30%. The most noticeable changes were observed in the relationship between the two daughter cells of each division. After cytokinesis, the intercellular contact area gradually increased during the following cell cycle in the 2/8- and 2/16-cell pairs, whereas it hardly changed in the 2/2- and 2/4-cell pairs. The comparison of the behaviour of the daughter cells on different substrates suggested that the zona pellucida and the mid body might have a role in the contact development at the early stages. Scanning electron microscopy was used for studying changes in the density of cell surface microvilli in an attempt to explain how the cells regulate their intercellular contacts.