Experiments were performed to observe the influence of X-ray irradiation on mast cells and mastocalcergy in rats. Animals were irradiated single dose of X-ray irradiation was applied to the whole body in doses either 100 rads or 150 rads(Cobalt-60 Teletheraphy Unit). One day after irradiation the rats were injected lead acetate intravenously, followed by injection of compound 48/80 in the back subcutaneously. Animals were killed by decapitation at intervals, 1 hour, 5 hours, 1 day and 6 days after subcutaneous injection. Specimens of the abdominal and back skin were fixed in alcoholformol solution and stained and stained with the following methods; H-E for observation of pathological changes of tissues, toluidine blue for demonstration of mast cells, von Kossa-azure A for demonstration of carbonate and phosphate, and chloranilic acid for demonstration of calcium. The following conclusions were obtained. Calciphylatic wheals are large size in the control group, medium size in 100 rads irradiation group and small size in 150 rads irradiation group. In X-ray irradiaion groups the number of mast cells decreases more in the 150 rads than in the 100 rads irradiation. In the 100 rads X-ray irradiation group, histochemical study of the injection sites showed that calcium impregnated to mast cell granules and connective tissue fibers in 1 days after subcutaneous injection. The morphogenesis of this calcinosis was the same in the rat of 5 hour after subcutaneous injection of the control group. Whereas, 1 day after subcutaneous inection in 150 rads X-ray irradiation group calcium deposited more slightly than other groups.