This report analyses the genetic underpinnings of the proportions of the hippocampal terminal fields in the mouse at the midseptotemporal level. We used 5 inbred strains and all possible F(1) crosses between them (diallel cross). Broad heritabilities ranged from 11 to 53%. Additive genetic variation was present for all phenotypes analyzed. Directional dominance was found for the relative size of the suprapyramidal mossy fiber terminal field only. For the stratum lacunosum-moleculare, ambidirectional dominance emerged. These findings suggest that, in evolutionary history, directional selection has operated for a proportionally large suprapyramidal terminal field. For all other hippocampal variables (viz. the relative sizes for the strata oriens, pyramidale, radiatum, lacunosum-moleculare, CA4, intra- and infrapyramidal mossy fiber terminal field and the absolute size of the regio inferior) past stabilizing selection was inferred.