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Abstract : Melatonin is the specific pineal hormone, that is secreted mainly at night. It ameliorates and stimulates the immune system function. Macrophages are connective tissue cells that are well known as one of the most important members in the immune system. This study was done to evaluate the effect of exogenous melatonin on thymic macrophages as an example of connective tissue cells. Adult male rats were supplied with dietary melatonin for 30 consecutive days. The animals were separated into 6 groups. The first group was designated as the control group, while groups two - six were given melatonin in a daily dose of 125, 250, 500, 750 and 1000 µg/kg body weight, correspondingly. At the end of the experiment, the left thymic lobe was dissected under anesthesia for subsequent histological study. The study showed stimulating effect of melatonin on the macrophages at its normal dosage, where as its effect was injurious and harmful at it overdosage. In conclusion, dietary melatonin had good effects on the rats’ thymic macrophages, on its normal small doses, but it has damaging effects on its large dose.