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EGIL HAQGENDAL, NILS JOHAN NILSSON and BENGT NORBACK. On the components of KrS5 clearance curves from the brain of the dog. Acta physiol.Scand. 1965. 66. Suppl. 258. 5-25. -Clearance curves from the brain in dogs were studied after short intra-arterial injections of Krs5. Multiexponential curves consisting of two main components were received both by recording y-and ,%radiation. The slopes of these two components corresponded to each other in the two types of curves. Local micro-injections of Krs5 in the cortical grey matter resulted in mono-exponential clearance curves with the same slope as the fast phase of the composite y-or /3-curve, whereas curves from micro-injections in the subcortical white matter had a slope corresponding to the slow phase of the composite curves. A third very slow phase of the composite y-curve was identified as due to flow in extracerebral tissue. In addition, an initial rapid phase was recorded in most curves after intraarterial injections and was assumed to be due to activity in the arterial blood. This phase 'was not present in clearance curves from blood drawn from the superior sagittal sinus, whereas the two main components representing blood flow in grey and white matter were identified. Some difficulties in the analysis of the composite clearance curves, especially concerning the slow component, are discussed.