SUMMARY Regional cerebral blood flow (rCBF) was simultaneously measured at seven locations in each hemisphere by the Obrist '"xenon inhalation method. In each of 35 healthy righthanded subjects two rest measurements were performed on consecutive days. The data analysis comprises the two-compartmentally derived parameters for flow (f,), relative tissue weight (w,), and fractional flow (FF,) respectively of the first compartment, and in addition the initial slope index (ISI). At each detector location the coefficient of variation (C.V.) of the change from first to second measurement was on average 10.4% (ISI), 14.2% (f,), 7.2% (w,), and 2.9% (FF,) respectively. However, when each regional measurement was expressed as a percentage of its hemispheric mean, the C.V. of the intermeasurement change was on average 4.4% (ISI), 7.0% (f,), 7.7% (w,), and 1.9% (FF,) respectively; that of the hemispheric means of ISI, f,, and FF, was found to be distinctly larger, whereas that of w, was about equal in size. The interhemispheric coefficient of variation for the change of the hemispheric means from first to second measurement was only 2.3% (ISI), 4.4% (f,), 1.6% (w,), and 1.1% (FF,) respectively.The findings suggest that (1) the variability of rCBF from subject -to subject and in consecutive measurements in the same subject is to a substantial degree of physiological origin, and that (2) there are two determinants of rCBF which may operate independently: a determinant of the hemispheric mean level, probably a single determinant for both hemispheres, and a set of determinants for each separate region superimposed on the hemispheric mean level.