Histology and morphometric changes in skin lesions of NMRI mice caused by dithranol and its acyl analogs, with and without pretreatment with dimethylbenzanthracene (DMBA) are described. The course of neoplastic progression seems to have three distinct phases (normal skin, hyperplasia, papilloma), each characterized by typical architectural and cellular changes. The parameters reflecting the nuclear size increased in the above order and the differences between the groups were highly significant. Nuclear shape changes did not take place, as shown by the almost constant form factor. The study demonstrates the value of the morphometric methodology in characterizing quantitative nuclear changes in mouse skin models.