Genes that produce anatomical brain differences between behaviorally "normal" inbred strains of house mice provide a chance to explore the functioning of intact nervous systems, as well as offering possible subtle precise alterations that are not experimentally achievable in any other manner. We are studying genetic variations in the number of granule cells of the dentate gyrus and pyramidal cells of regio superior of the hippocampus in house mice. Our work so far has demonstrated that the mouse Ammon's horn has at least five genetic divisions, and one size-dependent sex dimorphism has been established as well. A difference in two-way avoidance conditioning performance has been associated with a variation in granule cell density. This application proposes to continue the descriptive genetic, anatomical, and behavioral studies of Ammon's horn.