Hairless albino mice were fed aromatic retinoid in daily doses ranging from 1 to 50 mg/kg body weight. In animals receiving 1, 5, and 10 mg/kg, the thickness of the epidermis (vertical epidermal cross sectional areas) and the 3H-thymidine labeling indices were determined throughout a 3-week observation period. The following additional parameters were determined for animals fed 5 mg/kg; individual and cumulative cross sectional areas of epidermal cell nuclei and cytoplasms, nuclear-cytoplasm ratios, mean epidermal cell volume and mean number of epidermal cells. In animals fed 5 mg/kg for 7 days, an analysis of the cell kinetic parameters was performed including determination of duration of S and G2 phases, mean generation time and the proportion of noncycling (Go) cells. The changes induced by the aromatic retinoid were found to be dose dependent and consisted of a transitory cellular hypertrophy and a persistent numerical hyperplasia linked to increased labeling indices. The mean basal cell generation time was found to be greatly accelerated owing to a shortening of all cell cycle phases investigated.