A method is described for estimating the clearance rate and fractional blood flow of the fast (gray matter) compartment of the brain from the first ten minutes of 122 XE clearance curves, following a one-minute inhalation. Computer-simulated data were used to test the adequacy of the two-compartmental model employed, and to evaluate the stability of the parameters in the presence of random noise. A comparison was made between this approach and the previously reported three-compartmental analysis. Regional cerbral blood flow data were obtained on 48 young control subjects and 20 elderly demented patients. Hemispheral, regional, and test-retest variations were determined, as well as differences between the groups.