Brain metastases from primary or secondary breast tumors are difficult to model in the mouse. When metastatic breast cancer cell lines are injected directly into the arterial circulation, only a small fraction of cells enter the brain to form metastatic foci. To study the molecular and cellular mechanisms of brain metastasis, we have transfected MB-231BR, a brain-homing derivative of a human breast adenocarcinoma line MDA-MB-231, with the yellow fluorescent protein (YFP) variant Venus. MB-231BR selectively enters the brain after intracardiac injection into the arterial circulation, resulting in accumulation of fluorescent foci of cells in the brain that can be viewed by standard fluorescence imaging procedures. We describe how to perform the intracardiac injection and the parameters used to quantify brain metastasis in brain sections by standard one-photon fluorescence imaging. The disadvantage of this model is that the kinetics of growth over time cannot be determined in the same animal. In addition, the injection technique does not permit precise placement of tumor cells within the brain. This model is useful for determining the molecular determinants of brain tumor metastasis.