The intracerebral distribution of intravenously injected horseradish peroxidase (HRP) in young adult DDD mice was examined. HRP-tetramethylbenzidine reaction products were observed in the medial portions of the hippocampus, particularly the medial CA1 region and medial dentate gyrus. Reaction products were observed in the subfornical organ in mice decapitated 5 min after HRP injection, and then also progressively more caudally in the medial portions of the hippocampus as postinjection survival time increased. These findings suggest that blood-borne macromolecules have ready access to the medial portions of the hippocampus, particularly the medial CA1 region and medial dentate gyrus.