Temporal changes in brain uptake of 123I-labeled N-isopropyl-p-iodoamphetamine (IMP) were studied by means of a multi-detector single photon emission computed tomography (SPECT) scanner. Serial dynamic SPECT scan was performed for 50 min following an i.v. bolus injection of IMP in 30 patients with various neurologic diseases. In 18 of them the radioactivity in the lung was also recorded by a single probe detector during the serial dynamic SPECT scan. Brain activity showed a gradual increase after the injection of IMP, reaching 63 +/- 7% of the maximum activity at 5 min and 90 +/- 5% at 20 min. The lung clearance showed a large variation among the individual cases; 29 to 72% (52 +/- 11%) of the initial peak activity at 20 min and 22 to 57% (41 +/- 9%) at 50 min after the injection. A significant positive correlation was observed between the clearance half-time in the lung and the time to reach 90% of the maximum activity in the brain (r = 0.86, p less than 0.001). These results suggested that brain uptake of IMP is influenced by the lung clearance and the optimum time to start SPECT data acquisition using a conventional rotating gamma camera system is 20 min after the injection.