Longitudinal fiber systems in the hippocampal formation play an important role in propagating information to the entire septotemporal extent of the hippocampus. Previously, we have elucidated the longitudinal axonal projections of CA3 pyramidal cells (Ishizuka et al., 1990, J Comp Neurol 295: 580-623). Moreover, we have noted the existence of longitudinal fiber systems in the dentate gyrus. In the dentate gyrus, we found 2 types of longitudinal fiber systems originating from the hilar cells in the dentate gyrus-Type I and Type II fiber systems-by using the anterograde labeling technique with Phaseolus vulgaris-leucoagglutinin (PHA-L) and the retrograde labeling method with fluorescent dyes or the cholera toxin-B subunit. We observed that the Type I fiber system distributed its axonal ramifications in the inner molecular layer immediately above the granular cell layer, while the Type II fiber system distributed its terminals in the outer molecular layer. The Type I fiber system extended over almost the entire longitudinal (septotemporal) extent of the dentate gyrus, but in case of injection of the labeling molecule at the extreme temporal level, it extended within the temporal half of the dentate gyrus. In contrast, axonal arborizations of the Type II system were restricted within approximately 1.5 mm from the level of injection in each septotemporal direction. Axonal arborizations of the Type I fiber systems terminated densely in the narrow band of the inner molecular layer, while those of the Type II fiber system were loosely distributed in a slightly wider area of the outer molecular layer. The cells of origin of both Type I and Type II fiber systems were located in the hilar area. It appears likely that the cells of origin of the Type I system distributed their dendrites within the hilar area, since their cell bodies and axons were not labeled when the injection of PHA-L was restricted within the molecular layer. In contrast, the cells of origin of the Type II fiber system seemed to extend their dendrites over the outer molecular layer of the dentate gyrus. It appears likely that these longitudinal fiber systems of the dentate gyrus and the field CA3 function as an important system for analyzing information under physiological conditions, but under pathological conditions such as epileptic seizure abnormal neuronal bursting might expand to the entire extent of the hippocampus via these longitudinal systems.