X-irradiation of C3H/Bi mammary carcinomas with single exposures appeared to improve the colloidal-carbon filling of the existing microvasculature and cause a change in their morphologic appearance, with disappearance of some large-diameter sinusoidal vessels. Although the change was transient, the over-all microvascular picture after radiation treatment was similar to that of smaller nonirradiated tumors. Based upon these quantitative estimates, a transient capability for improved oxygen status conceivably existed for cells of the mouse carcinoma at various times after single-dose irradiation (particularly after 500 or 1500 R). A large single exposure of 4500 R, however, appeared to cause extensive damage to the microvasculature, as evidenced by a segmented pattern of vessel filling. To take full advantage of any reoxygenation in this tumor, the optimum time for a second exposure dose after 500 R might be 24 hr, while after 1500 R it might be 2-3 days.

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