In a comparative approach, the anatomical organization of the hippocampus was investigated in two species of megachiropteran bats, the grey-headed flying fox, Pteropus poliocephalus, and the little red flying fox, Pteropus scapulatus. In general, the cytoarchitectonic appearance of the flying fox hippocampus corresponded well with that of other mammals, revealing all major subdivisions. While the dentate fascia was trilaminated with a molecular layer, a granule cell layer, and a distinct polymorphic layer, the ammonic subfields were subdivided into stratum lacunosum molecular, stratum radiatum, stratum lucidum or mossy fiber layer (restricted to the CA3 region), pyramidal cell layer, and stratum oriens. In Ammon's horn, only subfields CA1, CA3, and CA3c were clearly discernible, whereas the CA2 region remained indistinct. In some cytoarchitectonic features, such as the dispersion of the pyramidal layer in CA1, the megachiropteran hippocampus resembled the corresponding region in primates. Five characteristic neuronal cell types of the megachiropteran hippocampus were studied in fixed slice preparations after intracellular injection with Lucifer Yellow. While the morphological appearance of CA3 pyramidal cells, horizontal stratum oriens cells, aspiny stellate cells, and mossy cells strongly resembled their counterparts in rodents, primates, and carnivores, granule cells showed an interesting variation from the nonprimate pattern. Like a subset of granule cells in the primate dentate gyrus, 75% of flying fox granule cells revealed 1-2 basal dendrites that ramified in the polymorphic layer. These processes are presumed to form the morphological substrate for recurrent excitation. Entorhinal afferents to Ammon's horn and the dentate fascia were revealed by employing the method of tract tracing in fixed tissue with the carbocyanine dye DiI. Similar to the rat and cat, but unlike the monkey, the entorhino-dentate projection in the flying fox is bilaminate, with medial entorhinal afferents occupying the middle third of the molecular layer and lateral entorhinal axons ramifying closer to the hippocampal fissure. The remaining inner third of the molecular layer was free from entorhinal input. In contrast to the radial organization of the projection to dentate gyrus and subfield CA3, entorhinal afferents to region CA1 followed a proximo-distal gradient, with medial entorhinal afferents terminating closer to the CA3/CA1 border. Photoconverted preparations were used to determine the trajectory of individual axons. The majority of entorhino-dentate axons traversed the hippocampal fissure, usually close to the crest region, and gave rise to several terminal branches with numerous en passant varicosities. Individual fibers coursed for considerable distances parallel to the granule cell layer, thus presumably activating a large number of postsynaptic granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)