Brain unidirectional extraction and flux of leucine were measured simultaneously with cerebral blood flow (CBF) at various times after transient global cerebral ischemia in the rat. The results permit an evaluation of blood-brain barrier permeability in the postischemic period independent of alter- ations in CBF at the time of measurement. Leucine extraction was higher (p< 0.001) than that of CBF- matched controls at 15 min and 6 hr after 30 min of global cerebral ischemia, but was not different from control at 30 min and 1 h after ischemia. Leucine flux into brain was increased only at 15 min after reperfusion of the brain.Cerebral edema occurs 15-30 min after reperfusion in this ischemia model, but the permeability of the blood-brain barrier to large molecules is unaltered during this period (Petito et al: J Neuropath Exp Neurol 41:423-436, 1982). Increased barrier permeability to small molecules such as leucine may contribute to the production of this early postischemic edema. Stroke Vol 15, No I. 1984