Hydroxyurea and cytarabine (cytosine arabinoside) interfere with DNA synthesis and mitosis in the epidermis and hair follicles of newborn rats and hairless mice. These effects take place more rapidly and cells recover faster after administration of hydroxyurea than after cytarabine. Both drugs induce necrotic nuclear changes which seem to be associated with the mitotic process. These findings and the time sequence of the cellular changes are of interest in view of recent reports of clinical management of psoriasis with hydroxyurea.