PURPOSE: Catheter-directed embolization has become a widespread technique for the treatment of benign and malignant neoplasms. The mechanism whereby embolization leads to selective atrophy of these neoplasms is largely speculative. As a potential model for the large regional perfusion differences between normal and neoplastic tissues, renal perfusion was studied before and after catheter-directed embolization. The working hypothesis was that embolization would create measurable changes in blood flow in the renal cortex and medulla. MATERIALS AND METHODS: Microspheres (l0 microm in diameter) containing a series of different fluorophores were injected into the arterial system before and after the renal arteries were embolized with a series of larger (100-300 microm) particulate embolic agents. The distribution of the microspheres in the renal cortex, renal medulla, and liver was analyzed by fluorescence microscopy as well as by extraction of the fluorophores. RESULTS: The distribution of the fluorescent microspheres was readily assessed by fluorescence microscopy or extraction of the fluorophores. Before embolization, the renal cortex received approximately three times more flow than the medulla. After embolization, perfusion of the renal cortex and medulla decreased in parallel. CONCLUSIONS: Fluorescent microspheres are a powerful tool for measuring the changes in flow that occur after catheter-directed embolization. The fact that parallel decreases in flow were found in the renal cortex and medulla indicates that the distribution of each embolic agent was flow-directed. These results might provide insight into the mechanism of tumor atrophy after uterine artery embolization or hepatic chemoembolization.