Peritoneal Macrophages (PMs) are specialized cells of the immune system that are responsible for ingesting dead cells and eliminating various pathogens. This study was performed on a total number of 82 rats (Rattus rattus norvegicus albenis) 41 of these rats included mature and 41 immature male rats. The peritoneal lavage was performed by injection of 5 ml of buffered phosphate saline (PH 7.4, 37 Cº) for each rat in the peritoneal cavity. The mean of the recovered volume was 4.5 ml. The PMs appeared as rounded cells with a pale eosinophilic cytoplasm, their nuclei were variable in shape. Morphological study of PMs was done by the use of different stains namely: Haematoxylin & Eosin and Lieshman’s stain. Morphometric analysis of PMs (was done by “Bel micro image Analyzer”) on smears of the lavage. Cytochemical study of PMs was performed by the identification of esteratic activity of the Alpha – naphthyl acetate esterase (ANAE) which demonstrated against the substrate α – naphthyl acetate (ANA), by the simultaneous azo- coupling method. The percentages of differential peritoneal lavaged cells count were composed of 66.3% PMs, 33.7% lymphocyte in mature rats, while in immature rats 59.3% PMs, 40.7% lymphocyte, it is clearly evident, that there is a significant differences in the ANAE ve+ and ANAE ve- between mature & immature rats. The results of surface area of PMs was 189.6 ± 54.5 µm2 in mature rats and 164.1 ± 43.7 µm2 in immature rats. There is a significant differences in the measurements of surface area between the two groups. The increase in the percentages of esterase positive cells in mature animals may be related to an age-related changes in the functions of alpha – naphthyl acetate esterases. The increase in the mean cell surface area of PMs that have been recognized in mature animals is related to increase exposure to environment of PMs that facilitate their antigen presentation.