We have developed a simple method to quantitate brain perfusion SPECT with N-isopropyl-p-[123I]iodoamphetamine (IMP). After an intravenous bolus injection of IMP, serial dynamic images of the brain and the lung were obtained every 5 sec for 2 min, followed by a SPECT scan at 20 min. Based on the assumption of IMP as chemical microspheres, fractional uptake (FU) of IMP in the brain was calculated from the time-activity curves of the total brain and the total lungs. FU showed a strong positive correlation with inverse of the body surface area which was calculated from the height and weight of the individual subject, suggesting that FU is influenced by cardiac output. Quantification of IMP brain perfusion SPECT was attempted with FU and estimated cardiac output in each subject, providing an index of regional cerebral blood flow. Compared with PET measurement of regional cerebral blood flow, the present method tended to overestimate the flow values. This method has a great advantage that it can be applied in many institutions having large field gamma cameras. However, the accuracy of the measurement may be limited in the tall subjects and in the cases with cardiopulmonary diseases, and further investigation is required to clarify the usefulness and limitation of this approach.