The numbers of basal cells, spinous cells and granular cells per high power vision field were recorded during regeneration of hairless mouse epidermis after topical cantharidin application. The mitotic rate was measured by a stathmokinetic method using Colcemid, and the transit time of 3H-TdR-labelled basal cells through the nucleated cell layers was estimated. A sequential increase in basal cell density, numbers of spinous cells and of granular cells indicates a transit time of about 20 h through the nucleated cell layers of regenerating epidermal cells. This transit time was confirmed by the movement of 3H-TdR pulse labelled cells through the nucleated cell layers, and is considerably shorter than the transit time of several days in normal mouse epidermis. Two periods of total loss of nucleated cells were seen; one between 20 h and 30 h after cantharidin application, and another after 40 h. Increased loss of nucleated epidermal cells means increased rate of terminal differentiation. How this relates to the loss of cornified cells from the surface remains to be investigated. Detailed knowledge about such loss is important for a more complete understanding of epidermal growth homeostasis during regeneration.