Abstract: This study was designed to evaluate changes in the number of mast cells in rats under different levels of protein deficiency. Twenty-four 20-day-old (immature group) and twenty-four 65-day-old (mature group) male Wistar Albino rats were used. They were divided into six experimental groups, each containing 8 animals. The small intestine tissue samples were obtained under deep anaesthesia. Intestine tissues were fixed in Mota’s fixative (Basic Lead Acetate - BLA) for 24 h and embedded in paraffin. Sections of 6 μm thickness were cut and stained with 0.5% toluidine blue in 0.5 N hydrochloric acid at pH 1.0 for 30 min. The numbers of mast cells in small intestine tissues were lower in immature rats fed 3% crude protein diet compared with, expect duodenum, rats fed 10% crude protein and control group (p>0.005). In mature rats, the numbers of mast cells were similar among fed 3% crude protein diet and control (p>0.05), but fewer than in the 10% crude protein (p>0.05).