Continuous carotid infusion of short-lived krypton-81m (t1/2 13 sec) yields an assessment of regional cerebral perfusion. This assessment can be obtained in three dimensions if activity is recorded with a rotating gamma camera and a computer to reconstruct krypton-81m distribution in tomographic sections. These showed several advantages over conventional views: (a) visualization of blood-flow distribution within brain structures (gray and white matter, basal ganglia); (b) more accurate location and evaluation of areas of relatively reduced or increased perfusion; (c) better definition of patterns of collateral circulation; (d) greater sensitivity and specificity in detecting and defining blood-flow changes during physiological activation studies. A limitation of the krypton-81m technique is its invasiveness. However, this study shows that the combination of new advances in radiochemistry with single-photon emission computed tomography may result in accessible methods for assessing, noninvasively and in three dimensions, the behavior of cerebral function in man.