Brains of western European hedgehogs, common tree shrews and common marmosets were perfused and stained according to Timm's method. The volumes of hippocampal components were determined with semiautomated quantitative techniques and compared by allometric methods. The results support previous findings that the size of the hippocampus in marmosets and tree shrews is about twice that of the hippocampus of the European hedgehogs of the same body weight. Three hippocampal substructures account for most of this progression. These are the hilus, the deep zone of the molecular layer of the area dentata, and CA1 (especially the cellular layer) of Ammon's horn.