The thinning of epidermis after urea treatment is explicable theoretically in several ways. We take the estimation of epidermal DNA synthesis measured autoradiographically by 3H-thymidine incorporation for a valid contribution to solve this question. As a result, the number of labeled nuclei in the stratum basale from ears of guinea pigs was decreased after a 5-day influence of urea down to half the number in untreated controls. Comparing the conclusions from these results and from biometric measurements, a further epidermal disturbance may easily be supposed in a lengthened life-time of postmitotic epidermal cells after contact with urea. There is no evidence whether these changes are either due directly to the influence of urea or only a secondary effect.