The effects of paired and brief trains of stimuli were compared in the medial and lateral components of the perforant pathway to the rat fascia dentata, both in vivo, and in the vitro transverse hippocampal slice. In the intact preparation a sharp transition in response properties occurred at the midpoint of the medial to lateral range of input. In vitro this transition was found to correspond exactly to the transition in Timm stainability which characterizes the border of the medial and lateral termination zones in the outer 2/3 of the molecular layer. These data indicate that there are two physiologically distinct subdivisions of the perforant path. Evidence is presented that the interpathway differences are at least partly due to differential quantal contents of the EPSP under resting conditions.