A method of measuring regional blood flow (RBF) that is simple in procedure and calculations is described. By arresting flow promptly after a short pulse of diffusible tracer, it is feasible to equate the tracer retained in the tissue (Cif) with that delivered by the blood. If the arterial pulse is characterized by its mean concentration (Ca) over a known duration (Δt), RBF can be estimated from Cif/CaΔt. The error involved is relatively small and can be corrected for. If the amount of tracer injected is known, this procedure also provides an estimate of cardiac output and its fractional distribution to the regions sampled. The values obtained for RBF in 4 regions of brain were similar to those previously reported.