The CA3 region of the hippocampus is an auto-associative network thought to underlie pattern completion. However, CA3 pyramidal cells also support major longitudinal connectivity along the septo-temporal axis, with individual axon collaterals extending for up to 75% of the entire septotemporal axis. This connectivity has been proposed to underlie integration of affective and spatial information in the ventral and dorsal hippocampal segments, respectively, but how this occurs during memory formation is unknown. We are using in-vivo imaging to examine the function of specific intrahippocampal CA3 projection systems at single-axon resolution during learning.