We examined blood flow changes and histology in the hippocampus induced by occlusion of the middle cerebral artery (MCA) by a filament in Swiss albino and SV-129 mice (n = 67) and in Wistar rats (n = 64). Filling cerebral arteries with carbon black revealed that one or both posterior communicating arteries were hypoplastic in 50% of Swiss mice. Ischemic changes were detected in the ipsilateral hippocampus with 2,3,5-triphenyl tetrazolium chloride or hematoxylin and eosin staining when these mice were subjected to 2-h MCA occlusion and 22-h reperfusion. No such abnormalities were found in SV-129 mice and Wistar rats (except one). The hippocampal blood flow dropped Co 60 +/-: 2.3% of the baseline in mice with a normal circle of Willis but to 37 +/- 4.2% in those with an incomplete circle when the MCA was occluded with a 6/0 nylon filament. When an 8/0 filament was used, no flow change in mice with a normal circle but a decrease to 60 +/- 2% in those with an incomplete circle was observed. A flow drop to 63 +/- 4% was also seen in Wistar rats when a 3/0 filament used. These data demonstrate that occlusion of the MCA by a thick filament may cause flow reduction in the hippocampus, which may be severe enough to lead to infarction if the circle of Willis is anomalous. (C) 1999 Published by Elsevier Science B.V. All rights reserved.