Mammary caicinoma was induced in rats by intravenous injection of N-methyl-N-Nitortourea. Microangiography was performed to evaluate the microvascular alterations in mammary carcinoma after irradiation. The tumors were given a single dose of 1,400 rads using Co-60 teletherapy unit with field size of 4$ imes$4cm at 40cm SSD. The dose rate was 147.5rads per minute. Microangiography was performed prior to irradiation and at one, two, and four weeks following irradiation. The results are as follows: 1. Before irradiation, mammary carcinoma in rats temded to form lobules and the basic vasculature consisted of peripheral vascular pattern with central peretrating vessels. The peripheral vascular pattern was always richer than that of the center. Irregular and tortuous vessles stretched from the periphery into the center of lobule. 2. One week following irradiation, an increase in the number of smaller, tortuous vessels and decreased intervascular distance were observed in the central port on of each lobule. This finding seems to be dut to and improved filling of some previously existing but unfilled vessels. This may lead to improved metabolic changes and reoxygenation. 3. Later changes of microvasculature after irradiation are tortuosity, irregularity, narrowing abrupt tapering, fragmentation, and extravasation. These findings progressed after a lapse of time. 4. The results can be considered as the microangiographic demonstration of the fact the reoxygenation after irradiation is mainly dut to dilatation of the collapsed tumor vessels.