The commissural projection to the displaced granule cells of the dentate gyrus in Reeler mutant mice has been examined with autoradiography, and light and electron microscopy. Commissural terminals in Reeler are confined to the hilar region, in contrast to normal littermates in which this projection is restricted to the inner part of the molecular layer. Granule cell somata in Reeler, but only exceptionally in normal littermates, are invested with spines, which have postsynaptic specializations, but no spine apparatus, and are contacted by presynaptic terminals. Between 20 and 30 h after destruction of the commissural fibres in Reeler, degenerating terminals can be found contacting both somatic and dendritic spines in the hilus; 30 h after decommissuration the number and length of spines on the somata and proximal dendrites of Golgi impregnated cells is greatly reduced, while spines on the distal parts of the dendritic tree are unaffected. A similar pattern of degeneration after decomissuration is found in the inner molecular layer of normal littermates. These results are discussed in terms of factors controlling the normal development of afferent projections. The paper concludes with an analysis of a potential methodological hazard. A change in mean spine length will of itself result in a change in the number of visible spines in golgi material. A quantitative assessment of the relation between spine length and the number of visible spines is developed for spherical cell bodies and cylindrical dendrites.