The organization of certain of the major afferents to the hippocampus and dentate gyrus has been studied in normal and reeler mutant mice using the autoradiographic and the anterograde degeneration methods. The distribution of the hippocampal and dentate afferents which arise in the medial and lateral parts of the entorhinal cortex and the hippocampus of both sides, has been found to be generally similar to that previously described in the rat, but there are a few minor differences that are discussed in the text. Despite the marked ectopia of many of the neurons in the hippocampal formation in the reeler mouse, the principal afferents to the hippocampus and the dentate gyrus maintain many of the features seen in normal mice. In particular, they maintain a normal radial sequence and a characteristic laminated and complementary arrangement. However, there are a number of significant differences in their distribution; for example, in the reeler mouse, the entorhinal afferents occupy the entire radial extent of the stratum moleculare of the dentate gyrus, whereas in normal mice they are restricted to the outer four-fifths of this layer. Furthermore, in the mutant the commissural and associational afferents to the dentate gyrus do not occupy the inner one-fifth of the molecular layer (as they do in normal animals) but rather are spread throughout the zone containing granule cells, which includes both the poorly-defined stratum granulosum and most of the hilar region of the dentate gyrus. Some of the developmental and functional implications of these and other abnormalities in the organization of the afferents to the hippocampus and dentate gyrus are discussed.