Metastasis is the leading cause of death in cancer patients. Cell motility is believed to be a necessary step in the metastatic process (L. Liotta and W. G. Stetler-Stevenson, In: Cancer: Principles and Practice of Oncology, pp. 134–149, 1993). Currently, most methods available to study the behavior of metastatic tumor cells are indirect, e.g., cell motility is examined in vitro and the results are correlated with metastatic capability (A. W. Partin, et al., Cancer Treat. Res., 59: 121–130, 1992). We have developed a model that directly examines the motility of metastatic primary tumor cells in situ. A metastatic rat breast cancer cell line was established that constitutively expresses green fluorescent protein. Upon s.c. injection of these cells into the mammary fat pad of female Fischer 344 rats, primary and metastatic tumors form that fluoresce when they are excited with FITC-filtered light. Animations of metastatic tumor cells moving in live rats were generated by intravital imaging of the primary tumor in situ on a laser scanning confocal microscope. With this model, the behavioral phenotype of metastatic and nonmetastatic tumor cells can be described and determined. This information will allow the effects of genetic manipulations or therapeutic treatments on this phenotype to be determined (D. R. Soll, Int. Rev. Cytol, 163: 43–104, 1995). This is the first time that living primary tumor cells in a live animal have been visualized as part of a clinically relevant model.