CBF obtained by the hydrogen clearance technique and cerebral blood volume (CBV) calculated from the [14C]dextran space were measured in three groups of rats subjected to temporary four-vessel occlusion to produce 15 min of ischaemia, followed by 60 min of reperfusion. In the control animals, mean CBF was 93 +/- 6 ml 100 g-1 min-1, which fell to 5.5 +/- 0.5 ml 100 g-1 min-1 during ischaemia. There was a marked early postischaemic hyperaemia (262 +/- 18 ml 100 g-1 min-1), but 1 h after the onset of ischaemia, there was a significant hypoperfusion (51 +/- 3 ml 100 g-1 min-1). Mean cortical dextran space was 1.58 +/- 0.09 ml 100 g-1 prior to ischaemia. Early in reperfusion there was a significant increase in CBV (1.85 +/- 0.24 ml 100 g-1) with a decrease during the period of hypoperfusion (1.33 +/- 0.03 ml 100 g-1). Therefore, following a period of temporary ischaemia, there are commensurate changes in CBF and CBV, and alterations in the permeability-surface area product at this time may be due to variations in surface area and not necessarily permeability.