The kinetic behaviour of the cranial neuroepithelial cells of rat embryos during neurulation is described. Serial transverse sections of 4-, 8-, 12- and 16-somite-stage embryos show that differential mitosis does not play a part in the mechanisms responsible for effecting cranial neural tube closure. A constant cell number is found in the midbrain/hindbrain neural epithelium during all four stages; the mitotic spindle axes are oriented parallel to the long axis of the embryo, so that increase in cell number occurs in this direction only. Growth is only expressed by an expansion in the volume of the forebrain, which projects rostral to the notochordal tip. [3H]thymidine studies (using an in vitro culture technique) show no significant variation in the cell cycle time between the forebrain and the midbrain/anterior hindbrain neural epithelium. It is suggested that the neural epithelium is a fluid structure whose overall shape is strictly controlled while the cells within it flow towards and into the rapidly expanding forebrain.