For the study of the normal epidermal layers of the stratum basale, stratum spinosum, stratum granulosum and (male rats at the age of 2 years) were used. Skin samples were obtained from the spinal area and processed by the method of transmission electron microscopy to get semithin and ultrathin sections. An electron microscopical examination preceded by an evaluation of the semithin sections in the light microscope showed that the epidermis of both age groups contained keratinocytes and Langerhans cells. Keratinocytes are typically arranged in epidermis layers of the stratum basale, stratum spinosum, stratum granulosum and stratum corneum. These layers are of different ultrastructure. The Langerhans cells are localized suprabasally in the epithelium. In the course of keratinization, prominent ultrastructural changes in keratinocytes occur. The cell organelles of keratinocytes including the nucleus disappear and the formation of specific cell components--prominent bundles of tonofilaments, keratohyalin granules and keratinosomes--can be observed. Keratinized cells of the stratum corneum exhibit a homogeneous or fibrillar inner structure. Langerhans cells contain characteristic cytoplasmic organelles--Langerhans granules. The epidermis of old rats differs from that of young animals in a special arrangement of the dermo-epidermal junction. The most typical change (in comparison with the epidermis of young rats) is the presence of multiplied lamina densa. The qualitative study of the normal epidermis of the laboratory rat is completed by morphometric analysis. The measurements concern keratinocytes of the stratum basale. Cell dimensions under the light microscope as well as some cell organelles in ultrathin sections are evaluated. Another part of the study is aimed to estimate morphological changes in the epidermis of rats which were percutaneously administered a toxic organophosphate. In this experiment, the tissue processing for electron microscopy could be realized only when the exposed epidermal area had been previously decontaminated. A group of intoxicated animals was therefore compared with a control group, the epidermis of which was affected by the decontaminating agent only. The ultrastructural changes noted in the epidermis of intoxicated animals show no specificity. Damaged membrane organelles and vacuoles in keratinocytes, dense cells, dilated epidermal intercellular spaces containing the material of medium density, etc. are present. Furthermore, the above-mentioned changes proved to be manifestation of a combined effect of the proper toxic substance and the decontaminating agent.