The Ammonic system of inbred mouse strains was examined with light and electron microscopic methods in order to determine whether genetically-associated variations occur in the distribution of dentate granule cell synapses (mossy fiber boutons) upon the dendrites of hippocampal pyramidal neurons in Regio Inferior. The results of this investigation showed that both qualitative and quantitative variations of the mossy fiber system occur among the strains examined. The most striking variation was seen in strain BALB/cJ. These mice are unique in that their mossy fibers do not form an appreciable infrapyramidal synaptic field, but form an extensive intrapyramidal synaptic field instead. In marked contrast, SM/J mice exhibit an extensive development of infrapyramidal mossy fiber boutons while their intrapyramidal mossy fiber synaptic field, when present at all, is limited to a very small portion of Regio Inferior adjacent to the hilus of Area Dentata.Since the mouse strains used in this study do not exhibit neurological defects (i.e., they are not neurological mutants identified through the presence of gross behavioral or external anatomical abnormalities), the described variations may be thought of as representing genotype-dependent differences in the connectivity of the Ammonic system which might occur within a "normal," heterogeneous population of mice. Genetic, developmental and functional implications of the observed variations in the distribution of mossy fiber boutons upon Regio Inferior pyramids are discussed.Genetically-associated variations in the connectivities between specific neuronal populations promise to increase our understanding of how neural systems function as well as how they develop unique connections during ontogeny. Ammon's Horn (hippocampus and dentate gyrus) is amenable to a search for subtle and precise structural variability in mammalian CNS because it is a highly organized, laminar structure about which there is considerable knowledge concerning its afferent supply, intrinsic connectivity and efferent projec-

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