Abstract 1. 1. A study has been made of the uptake of tritiated thymidine by slices of guinea pig ear skin incubated in culture medium for four hours after removal from the animal. During this period, 7–8 per cent of the epidermal cells become labelled. 2. 2. The infliction of micro-wounds in the epidermis before removal of the skin slices causes an increase in the number of positive cells as soon as four hours after injury, reaching a maximum of 30 per cent at 24 hr. The increase is discontinuous, and consists of cycles of high and low percentage positivity at 8–12 hr intervals. 3. 3. This indicated that the earliest effect of injury is to cause an increase in the number of DNA synthesizing cells, to impose a temporary synchrony upon cell division, and to shorten the cell cycle to 8–12 hr. 4. 4. A study of the uptake of tritiated thymidine by skin slices after various intervals in culture shows that, after removal from the animal, the number of cells taking up thymidine decreases to a minimum of 1–2 per cent at 16 hr, with a subsequent tendency to recover to normal values at 24–48 hr. 5. 5. The infliction of microwounds in the epidermis after removal of the skin slice from the animal causes no stimulus to DNA synthesis, although migration of epidermal cells takes place normally.