In an experiment on the spleen of healthy lambs at two periods (prenatal and postnatal), using anatomo-topographic and histo-cytological research methods and with subsequent statistical analysis, it was found that the changes in the anatomical and histological structure related to the age of this organ, including during the early stages of the postnatal period of ontogenesis. Note that the surface of the spleen is covered with a capsule of connective tissue, from which leave trabeculae which divide the parenchyma of the organ into lobes. At the time of physiological maturity, the ratio of the structures changes in the direction of an increase in the white pulp and a decrease in the component of the red pulp. During all periods of prenatal fetal development, the percentages of spleen tissue components corresponding to fetal analogs with normal clinical development are determined. The morphological structure of the spleen is described for the first time for this breed in Algeria, age-related changes in the structures of the parenchyma are traced, the formation of follicles began during the terminal phase of gestation, and the development age-related white pulp.