Tumor blood flow is an important determinant of the efficacy of presently available antineoplastic treatment modalities. Using 113Sn-labeled microspheres, 25 micron in diameter, we measured blood flow to primary tumors and regional lymph node metastases in conscious SMT-2A mammary adenocarcinoma-bearing syngeneic rats following a single i.v. bolus injection of the calcium entry blocker, flunarizine. Tumor blood flow increased in a biphasic dose-dependent fashion; at a dose of 1 mg/kg, primary tumor blood flow increased approximately 50% (p less than 0.001) without a significant change in heart rate or blood pressure. The increase in flow was distributed to both the peripheral viable and central necrotic regions of the tumor and was still detectable 45 min following administration of the drug. A similar increase in blood flow was demonstrated for lymph node metastases (p less than 0.001).