We have studied pre-mitotic nuclear migration in living subsidiary mother cells (SMCs) of Tradescantia virginiana. Divisions in the four SMCs of each stomatal complex are asymmetrical and are preceded by the migration of nuclei from random locations in the cells to positions adjacent to the central guard mother cells (GMCs). In newly polarised SMCs, nuclei display erratic movements which gradually dampen over time. In older complexes, where nuclear migration occurred earlier in the ontogeny of the leaf, nuclei are stably positioned and change in morphology from spherical to dome-shaped. Labelling with bromodeoxyuridine (BrdU) shows that SMCs polarised in G1 of the cell cycle and remain polarised for a minimum of 22 h before entering mitosis, while the inducing GMCs stay in G1. Centrifugation (1,320 g, 15 min) of epidermal peels displaces the majority of nuclei to the centrifugal end of cells, including nuclei in polarised SMCs. After centrifugation, most SMC nuclei return towards the GMCs within 100 min in control and 5 microM oryzalin treated peels. However, nuclei are unable to reposition in the presence of cytochalasin B (5 micrograms/ml). Thus, the signal for SMC polarisation is issued very early in the ontogeny of the cells (G1), is apparently sustained for a prolonged period, and results in the actin-dependent migration of nuclei towards the GMC. Cytological changes and nuclear migrations similar to those occurring in polarising SMCs can be induced by a local application of pressure to the surface of epidermal cells.