This study sought to determine whether the DG, CA3, and CA1 regions contain uniformly excitable populations and test the hypothesis that rapid addition of new, more excitable, granule cells in prepubescence results in a low activation probability (P1) in the DG. The immediate-early gene Homer1a was used as a neural activity marker to quantify activation in juvenile (P28) and adult (~5 mo) rats during track running. The main finding was that P1 in juveniles was substantially lower not only the DG, but also CA3 and CA1. The P1 for a DG granule cell was close to 0 in juveniles, versus 0.58 in adults. The low P1 in juveniles indicates that sparse, but non-overlapping, subpopulations participate in encoding events. Since sparse, orthogonal coding enhances a network’s ability to decorrelate input patterns (Marr, 1971; McNaughton & Morris, 1987), the findings suggest that juveniles likely possess greatly enhanced pattern separation ability. / ix, 51 leaves : ill. ; 29 cm