Metastatic brain tumor model. A) One hundred thousand 125I-deoxyuridine-labeled B16F10 cells were injected in the left internal carotid artery. Mice were sacrificed after 1, 24, and 48 hours, 3 days and 7 days. The left hemisphere was removed and the amount of radioactivity was measured by means of the ICN gamma-counter to determine the number of cells remaining in the brain. Note the rapid decline of radioactivity in the brain that remained stable after 72 hours. Hematoxylin-eosin staining of melanoma tumors developing in the brain after intracarotid injection (B-D). B) B16F10 cells lodged in a blood vessel (arrowhead) and cells invading the brain (arrow). C) B16F10 cells (arrow) growing in the brain parenchyma. D) Large tumor growing in the brain parenchyma (arrow).