We have investigated the distribution and organization of the projection from the hippocampal formation to the nucleus accumbens in the rat. In the first experiments, horseradish peroxidase was injected into the fimbria fornicis. This procedure resulted in massive anterograde labelling of fornix fibers, and enabled the hippocampo--accumbens projection to be charted in detail. Labelled fornix fibers are distributed to the entire length of the nucleus accumbens and do not spread lateral to the anterior limb of the anterior commissure, that is, the projection is limited to the medial nucleus accumbens. Terminal labelling is particularly dense in a caudal dorsomedial sector of accumbens immediately adjacent to the septum. In the second part of the study, horseradish peroxidase was microelectrophoretically injected into the nucleus accumbens, in order to confirm the findings from the anterograde experiments. When the deposit was localized within the fornix distribution field, as indicated by the first experiment, retrogradely-labelled cells were observed in the subiculum, prosubiculum, and to a lesser extent, in hippocampal field CAI. When the deposit was located outside of the fornix distribution field, no hippocampal labelling was noted. The topography of the projection, suggested by the retrograde labelling experiments, is discussed. The findings confirm earlier reports of such a projection from the hippocampal formation to the nucleus accumbens, and with the use of a relatively novel anterograde-horseradish peroxidase technique, provide a picture of fiber labelling representing the entire field of origin of the fornix system. The striatal projection field of this pathway is discussed in relation to other striatal afferents, with particular emphasis on limbic afferentation of the striatum. Finally, the so-called 'hippocampal district' of striatum is discussed with respect to its neurotransmitter composition, as well as its functional importance regarding limbic system influence on central motor mechanisms.