A patient with pheochromocytoma was studied by MIBG scintigraphy while on labetalol therapy, which has been reported to interfere with imaging. Serial imaging was performed at 2, 18, 25, and 90 hours to obtain time/activity curves. Blood samples were taken at each imaging study to determine activity counts. Tumor activity per gram of tissue (surgery 116 hours after injection) was compared with blood activity. Tumor and blood activity decreased concurrently, whereas the decrease in peritumoral activity was faster, thus providing transiently improved contrast on 18- and 25-hour images. The diagnostic implications of these results are discussed.